Add documentation to migrate from Dagger/Anvil to Metro

Author: bmarty

docs/migration_to_metro.md

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+# Migration to Metro
+
+The dependency injection library is now [Metro](https://zacsweers.github.io/metro/latest/). It replaces both Dagger and Anvil.
+
+Migration of the current Element X code has been performed in https://github.com/element-hq/element-x-android/pull/5253.
+
+To migrate other existing code you will need to:
+
+- replace `setupAnvil()` with `setupDependencyInjection()` in your `build.gradle.kts` files
+- replace the Dagger and Anvil imports with Metro ones
+- move the `@Inject` apply to the constructor to the class itself (only applicable if there is only one primary constructor
+- replace `@AssistedInject` with `@Inject`
+- replace `@Module` with `@BindingContainer`
+
+This should help to migrate your existing code.