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/injectable.dart

@@ -39,5 +39,6 @@ import 'package:meta/meta.dart';
 /// After running the generator, the mixin (`_\$ProfileScreen`) will be available to help auto-inject all [@inject] fields in your widget/service/controller.
 @experimental
 final class injectable {
+  /// Creates an [injectable] annotation for classes.
   const injectable();
 }