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/provide.dart

@@ -39,6 +39,6 @@ import 'package:meta/meta.dart';
 /// See also: [@singleton], [@instance], [@params], [@named]
 @experimental
 final class provide {
-  /// Creates a [provide] annotation.
+  /// Creates a [provide] annotation for marking provider methods/classes in DI modules.
   const provide();
 }