## 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
