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/instance.dart

@@ -45,5 +45,6 @@ import 'package:meta/meta.dart';
 /// See also: [@singleton]
 @experimental
 final class instance {
+  /// Creates an [instance] annotation for classes or providers.
   const instance();
 }