docs(annotations): improve API documentation and usage example

- Add detailed English doc comments for all main annotations (inject, injectable, instance, provide, scope, etc)
- Add fully documented example/example.dart illustrating real-world DI scenario
- Clarify stub sections (Module class, generated mixins)
- Aligns package with pub.dev quality and best practice requirements

No breaking changes.

cherrypick_annotations/lib/src/inject.dart

@@ -38,5 +38,6 @@ import 'package:meta/meta.dart';
 /// ```
 @experimental
 final class inject {
+  /// Creates an [inject] annotation for field injection.
   const inject();
 }