Fix broken doc links (#2046)

## What

This PR fixes some broken links that have been hanging around in the
docs for what seems like a very long time.

## Why

As discovered by the link check in #1912, there are a fair few broken
links in the docs. Everyone hates broken links because it makes it
harder to understand what they were supposed to be pointing at.

## How

There are 3 types of links that have been fixed:
- Links that should have been internal but were absolute. E.g.
  `https://docs.getunleash.io/path/article` that should have been
  `./article.md`
- External links that have changed, such as Slack's API description
- GitHub links to files that either no longer exist or that have been
  moved. These links generally pointed to `master`/`main`, meaning
  they are subject to change. They have been replaced with permalinks
  pointing to specific commits.

-----

* docs: fix slack api doc link

* docs: update links in migration guide

* docs: fix broken link to ancient feature schema validation

* docs: update links to v3 auth hooks

* docs: update broken link in the go sdk article

* Fix: use permalink for GitHub link

* docs: fix wrong google auth link

website/docs/deploy/configuring-unleash-v3.md

@@ -60,7 +60,7 @@ unleash.start(unleashOptions);
   - `custom` - use this when you implement your own custom authentication logic.
 - **ui** (object) - Set of UI specific overrides. You may set the following keys: `headerBackground`, `environment`, `slogan`.
 - **getLogger** (function) - Used to register a [custom log provider](#how-do-i-configure-the-log-output).
-- **eventHook** (`function(event, data)`) - If provided, this function will be invoked whenever a feature is mutated. The possible values for `event` are `'feature-created'`, `'feature-updated'`, `'feature-archived'`, `'feature-revived'`. The `data` argument contains information about the mutation. Its fields are `type` (string) - the event type (same as `event`); `createdBy` (string) - the user who performed the mutation; `data` - the contents of the change. The contents in `data` differs based on the event type; For `'feature-archived'` and `'feature-revived'`, the only field will be `name` - the name of the feature. For `'feature-created'` and `'feature-updated'` the data follows a schema defined in the code [here](https://github.com/Unleash/unleash/blob/master/lib/routes/admin-api/feature-schema.js#L38-L59). See an [api here](/api/admin/events).
+- **eventHook** (`function(event, data)`) - If provided, this function will be invoked whenever a feature is mutated. The possible values for `event` are `'feature-created'`, `'feature-updated'`, `'feature-archived'`, `'feature-revived'`. The `data` argument contains information about the mutation. Its fields are `type` (string) - the event type (same as `event`); `createdBy` (string) - the user who performed the mutation; `data` - the contents of the change. The contents in `data` differs based on the event type; For `'feature-archived'` and `'feature-revived'`, the only field will be `name` - the name of the feature. For `'feature-created'` and `'feature-updated'` the data follows [this schema defined in the source code](https://github.com/Unleash/unleash/blob/a06d2c04bb0d83d9c8c5bf2a90e9dace50f0b10a/lib/routes/admin-api/feature-schema.js#L38-L59). See an [api here](/api/admin/events).
 - **baseUriPath** (string) - use to register a base path for all routes on the application. For example `/my/unleash/base` (note the starting /). Defaults to `/`. Can also be configured through the environment variable `BASE_URI_PATH`.
 - **unleashUrl** (string) - Used to specify the official URL this instance of Unleash can be accessed at for an end user. Can also be configured through the environment variable `UNLEASH_URL`.
 - **secureHeaders** (boolean) - use this to enable security headers (HSTS, CSP, etc) when serving Unleash from HTTPS. Can also be configured through the environment variable `SECURE_HEADERS`.

website/docs/deploy/migration-guide.md

@@ -42,7 +42,7 @@ const user = userService.loginUserWithoutPassword(
 req.session.user = user;
 ```
 
-- [Read more about Securing Unleash v4](./securing_unleash)
+- [Read more about Securing Unleash v4](./securing-unleash.md)
 - [Read more about RBAC](../user_guide/rbac)
 
 ### 4. Legacy v2 routes removed {#4-legacy-v2-routes-removed}
@@ -59,18 +59,17 @@ Read more in our [getting started documentation](./getting_started)
 
 ## Upgrading from v2.x to v3.x {#upgrading-from-v2x-to-v3x}
 
-The notable change introduced in Unleash v3.x is a strict separation of API paths for client requests and admin requests. This makes it easier to implement different authentication mechanisms for the admin UI and all unleash-clients. You can read more about [securing unleash](https://github.com/Unleash/unleash/blob/master/docs/securing-unleash.md).
+The notable change introduced in Unleash v3.x is a strict separation of API paths for client requests and admin requests. This makes it easier to implement different authentication mechanisms for the admin UI and all unleash-clients. You can read more about [securing unleash](./securing-unleash.md).
 
 The recommended approach is to first upgrade the `unleash-server` to v3 (which still supports v2 clients). After this is done, you should upgrade all your clients to v3.
 
-After upgrading all your clients, you should consider turning off legacy routes, used by v2 clients. Read more about this option in the [Getting started guide](https://github.com/Unleash/unleash/blob/master/docs/getting-started.md#2-or-programmatically).
+After upgrading all your clients, you should consider turning off legacy routes, used by v2 clients. To do this, set the configuration option `enableLegacyRoutes` to `false` as described in the [page on configuring Unleash v3](./configuring-unleash-v3.md).
 
 ## Upgrading from v1.0 to v2.0 {#upgrading-from-v10-to-v20}
 
 ### Caveat 1: Not used db-migrate to migrate the Unleash database? {#caveat-1-not-used-db-migrate-to-migrate-the-unleash-database}
 
-In FINN we used liquibase, for internal reasons, to migrate our database.  
-Because unleash from version 2.0 migrates the database internally, with db-migrate, you need to make sure that all previous migrations for version 1 exist, so that Unleash does not try to create already existing tables.
+In FINN we used liquibase, for internal reasons, to migrate our database. Because unleash from version 2.0 migrates the database internally, with db-migrate, you need to make sure that all previous migrations for version 1 exist, so that Unleash does not try to create already existing tables.
 
 #### How to check? {#how-to-check}
 

website/docs/deploy/securing-unleash-v3.md

@@ -25,7 +25,7 @@ unleash
     adminAuthentication: 'custom',
     preRouterHook: myCustomAdminAuth,
   })
-  .then(unleash => {
+  .then((unleash) => {
     console.log(
       `Unleash started on http://localhost:${unleash.app.get('port')}`,
     );
@@ -43,9 +43,9 @@ Additionally, you can trigger the admin interface to prompt the user to sign in
 
 Examples of custom authentication hooks:
 
-- [google-auth-hook.js](https://github.com/Unleash/unleash/blob/master/examples/google-auth-hook.js)
-- [basic-auth-hook.js](https://github.com/Unleash/unleash/blob/master/examples/basic-auth-hook.js)
-- [keycloak-auth-hook.js](https://github.com/Unleash/unleash/blob/master/examples/keycloak-auth-hook.js)
+- [google-auth-hook.js](https://github.com/Unleash/unleash-examples/blob/7ed25f97a31dfd8f773c00847080b1a4c889fd87/v3/securing-google-auth/google-auth-hook.js)
+- [basic-auth-hook.js](https://github.com/Unleash/unleash-examples/blob/7ed25f97a31dfd8f773c00847080b1a4c889fd87/v3/securing-basic-auth/basic-auth-hook.js)
+- [keycloak-auth-hook.js](https://github.com/Unleash/unleash-examples/blob/7ed25f97a31dfd8f773c00847080b1a4c889fd87/v3/securing-keycloak-auth/keycloak-auth-hook.js)
 
 We also have a version of Unleash deployed on Heroku which uses Google OAuth 2.0: https://secure-unleash.herokuapp.com
 
@@ -73,7 +73,7 @@ const sharedSecret = '12312Random';
 unleash
   .start({
     databaseUrl: 'postgres://unleash_user:passord@localhost:5432/unleash',
-    preRouterHook: app => {
+    preRouterHook: (app) => {
       app.use('/api/client', (req, res, next) => {
         if (req.header('authorization') !== sharedSecret) {
           res.sendStatus(401);
@@ -83,11 +83,11 @@ unleash
       });
     },
   })
-  .then(unleash => {
+  .then((unleash) => {
     console.log(
       `Unleash started on http://localhost:${unleash.app.get('port')}`,
     );
   });
 ```
 
-[client-auth-unleash.js](https://github.com/Unleash/unleash/blob/master/examples/client-auth-unleash.js)
+[client-auth-unleash.js](https://github.com/Unleash/unleash-examples/blob/7ed25f97a31dfd8f773c00847080b1a4c889fd87/v3/securing-client-auth/index.js)

website/docs/guides/feature-updates-to-slack.md

@@ -4,7 +4,9 @@ title: Feature Updates To slack
 ---
 
 :::caution
+
 This guide is deprecated. If you're looking for ways to integrate with Slack, you should refer to [the Slack add-on guide](../addons/slack.md) instead.
+
 :::
 
 ## Create a custom Slack WebHook url: {#create-a-custom-slack-webhook-url}
@@ -12,7 +14,7 @@ This guide is deprecated. If you're looking for ways to integrate with Slack, yo
 1. Go to [https://slack.com/apps/manage/custom-integrations](https://slack.com/apps/manage/custom-integrations)
 1. Click Incoming WebHooks
 1. Click “Add Configuration”
-1. This is Slack's help page on how to do this: https://api.slack.com/custom-integrations/incoming-webhooks
+1. This is Slack's help page on how to do this: https://api.slack.com/messaging/webhooks
    - Choose a channel, follow the wizard, get the custom URL.
 
 ## Send data to Slack using an event hook function {#send-data-to-slack-using-an-event-hook-function}
@@ -63,10 +65,10 @@ function onEventHook(event, eventData) {
         text: text,
       },
     )
-    .then(res => {
+    .then((res) => {
       console.log(`Slack post statusCode: ${res.status}. Text: ${text}`);
     })
-    .catch(error => {
+    .catch((error) => {
       console.error(error);
     });
 }
@@ -75,7 +77,7 @@ const options = {
   eventHook: onEventHook,
 };
 
-unleash.start(options).then(server => {
+unleash.start(options).then((server) => {
   console.log(`Unleash started on http://localhost:${server.app.get('port')}`);
 });
 ```

website/docs/sdks/go.md

@@ -69,7 +69,7 @@ Read more about the strategies in [the activation strategies document](../user_g
 
 ### Unleash context {#unleash-context}
 
-In order to use some of the common activation strategies you must provide a [unleash-context](https://github.com/Unleash/unleash/blob/master/docs/unleash-context.md). This client SDK allows you to send in the unleash context as part of the `isEnabled` call:
+In order to use some of the common activation strategies you must provide an [_Unleash context_](../user_guide/unleash-context.md). This client SDK allows you to send in the unleash context as part of the `isEnabled` call:
 
 ```go
 ctx := context.Context{