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/scope.dart

@@ -49,5 +49,7 @@ import 'package:meta/meta.dart';
 final class scope {
   /// The name/key of the DI scope from which to resolve this dependency.
   final String? name;
+
+  /// Creates a [scope] annotation specifying which DI scope to use for the dependency resolution.
   const scope(this.name);
 }