docs(www): add v21 migrations

Author: timdeschryver

projects/www/src/app/pages/guide/migration/v21.md

@@ -20,3 +20,52 @@ Version 21 has the minimum version requirements:
 - RxJS version ^6.5.x || ^7.5.0
 
 ## Breaking changes
+
+### @ngrx/signals
+
+#### rename `withEffects` to `withEventHandlers`
+
+The `withEffects` feature from the `@ngrx/signals/events` plugin is renamed to `withEventHandlers`.
+
+BEFORE:
+
+```ts
+import { withEffects } from '@ngrx/signals/events';
+
+export const CounterStore = signalStore(
+  withState({ count: 0 }),
+  withEffects((store, events = inject(Events)) => ({
+    logCount$: events
+      .on(increment)
+      .pipe(tap(() => console.log(store.count()))),
+  }))
+);
+```
+
+AFTER:
+
+```ts
+import { withEventHandlers } from '@ngrx/signals/events';
+
+export const CounterStore = signalStore(
+  withState({ count: 0 }),
+  withEventHandlers((store, events = inject(Events)) => ({
+    logCount$: events
+      .on(increment)
+      .pipe(tap(() => console.log(store.count()))),
+  }))
+);
+```
+
+### @ngrx/eslint-plugin
+
+The lint rules that require type information have been moved to seperate predefined configurations.
+
+| Configuration    | Configuration including type checked rules |
+| ---------------- | ------------------------------------------ |
+| `all`            | `allTypeChecked`                           |
+| `componentStore` | N/A                                        |
+| `effects`        | `effectsTypeChecked`                       |
+| `operators`      | N/A                                        |
+| `signals`        | `signalsTypeChecked`                       |
+| `store`          | N/A                                        |