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/cherrypick_annotations.dart

@@ -1,5 +1,3 @@
-library;
-
 //
 // Copyright 2021 Sergey Penkovsky (sergey.penkovsky@gmail.com)
 // Licensed under the Apache License, Version 2.0 (the "License");
@@ -13,6 +11,24 @@ library;
 // limitations under the License.
 //
 
+/// Annotations for use with the CherryPick dependency injection generator.
+///
+/// These annotations are used on classes, methods, fields or parameters to
+/// describe how they should participate in dependency injection.
+/// See: https://pub.dev/packages/cherrypick
+///
+/// Example:
+/// ```dart
+/// import 'package:cherrypick_annotations/cherrypick_annotations.dart';
+///
+/// @injectable()
+/// class MyService {
+///   @inject()
+///   late final Logger logger;
+/// }
+/// ```
+library;
+
 export 'src/module.dart';
 export 'src/provide.dart';
 export 'src/instance.dart';