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/inject.dart

@@ -38,5 +38,6 @@ import 'package:meta/meta.dart';
 /// ```
 @experimental
 final class inject {
+  /// Creates an [inject] annotation for field injection.
   const inject();
 }

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();
 }

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();
 }

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();
 }

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);
 }