docs: suggest to use strategy constraints instead of custom strategies (#4215)

## About the changes
Custom activation strategies are still the recommended way to go in our
documentation, but now most of the use cases for custom activation
strategies can be covered by strategy constraints (released with Unleash
4.16)

With this change, we try to drive people to use strategy constraints.

---------

Co-authored-by: Thomas Heartman <thomas@getunleash.ai>

website/docs/how-to/how-to-use-custom-strategies.md

@@ -2,6 +2,14 @@
 title: How to use custom activation strategies
 ---
 
+:::tip
+
+Use custom activation strategies only for cases where [strategy constraints](https://docs.getunleash.io/reference/strategy-constraints) don't give you enough control. 
+
+The downside of using a custom activation strategy instead of constraints is that you need to distribute the code of your strategy with Unleash client SDK while on the other hand, [strategy constraints](https://docs.getunleash.io/reference/strategy-constraints) work without any extra code or maintenance.
+:::
+
+
 This guide takes you through how to use [custom activation strategies](../reference/custom-activation-strategies.md) with Unleash. We'll go through how you define a custom strategy in the admin UI, how you add it to a toggle, and how you'd implement it in a [client SDK](../reference/sdks/index.md).
 
 In this example we want to define an activation strategy offers a scheduled release of a feature toggle. This means that we want the feature toggle to be activated after a given date and time.

website/docs/reference/activation-strategies.md

@@ -4,7 +4,7 @@ title: Activation Strategies
 
 It is powerful to be able to turn a feature on and off instantaneously, without redeploying the application. Activation strategies let you enable a feature only for a specified audience. Different strategies use different parameters. Predefined strategies are bundled with Unleash. The recommended strategy is the gradual rollout strategy with 100% rollout, which basically means that the feature should be enabled to everyone.
 
-Unleash comes with a number of built-in strategies (described below) and also lets you add your own [custom activation strategies](../reference/custom-activation-strategies.md) if you need more control. However, while activation strategies are _defined_ on the server, the server does not _implement_ the strategies. Instead, activation strategy implementation is done client-side. This means that it is _the client_ that decides whether a feature should be enabled or not.
+Unleash comes with a number of built-in strategies (described below) that can be enhanced with [constraints](https://docs.getunleash.io/reference/strategy-constraints) for fine-grained control. For more advanced use cases, where constraints do not fulfill your needs, you can add your own [custom activation strategies](../reference/custom-activation-strategies.md). However, while activation strategies are _defined_ on the server, the server does not _implement_ the strategies. Instead, activation strategy implementation is done client-side. This means that it is _the client_ that decides whether a feature should be enabled or not.
 
 All [server-side client SDKs](../reference/sdks/index.md#server-side-sdks) and the [Unleash Proxy](../generated/unleash-proxy.md) implement the default strategies (and allow you to add your own [custom strategy implementations](../reference/custom-activation-strategies.md#implementation)). The [front-end client SDKs](../reference/sdks/index.md#front-end-sdks) do not do the evaluation themselves, instead relying on the [Unleash Proxy](../generated/unleash-proxy.md) to take care of the implementation and evaluation.
 

website/docs/reference/custom-activation-strategies.md

@@ -6,6 +6,8 @@ title: Custom Activation Strategies
 
 This document is a reference for custom activation strategies. If you're looking for a guide on how to use them, see the [_how to use custom strategies_ guide](../how-to/how-to-use-custom-strategies.md).
 
+Custom activation strategies should be considered an advanced feature. In most cases [strategy constraints](https://docs.getunleash.io/reference/strategy-constraints) provides enough control without the additional complexity.
+
 :::
 
 **Custom activation strategies** let you define your own activation strategies to use with Unleash. When the [built-in activation strategies](../reference/activation-strategies.md) aren't enough, custom activation strategies are there to provide you with the flexibility you need.