WIP: Extract remaining inline code from getting_started.md (2/3)

Extracted code from:
- getting_started.md: Main counter example + mixin example

Created:
- mixin_simple_example.dart

Reused:
- counter_simple_example.dart (already had all necessary regions)

All examples compile successfully.

Still TODO: watching_streams_and_futures, handlers, watching_widgets,
watch_ordering_rules, lifecycle, observing_commands, best_practices,
debugging_tracing, advanced_integration (100+ inline code blocks)

Author: escamoteur

code_samples/lib/watch_it/mixin_simple_example.dart

@@ -0,0 +1,20 @@
+import 'package:flutter/material.dart';
+import 'package:watch_it/watch_it.dart' hide di;
+import '_shared/stubs.dart';
+
+// #region example
+class MyWidget extends StatelessWidget with WatchItMixin {
+  const MyWidget({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final data = watchValue((DataManager m) => m.data);
+    return Text('$data');
+  }
+}
+// #endregion example
+
+void main() {
+  setupDependencyInjection();
+  runApp(MaterialApp(home: MyWidget()));
+}

docs/documentation/watch_it/getting_started.md

@@ -35,39 +35,7 @@ dependencies:
 
 Here's a simple counter that rebuilds automatically when the count changes:
 
-```dart
-// 1. Create a manager with reactive state
-class CounterManager {
-  final count = ValueNotifier<int>(0);
-
-  void increment() => count.value++;
-}
-
-// 2. Register it in get_it
-void main() {
-  di.registerSingleton<CounterManager>(CounterManager());
-  runApp(MyApp());
-}
-
-// 3. Watch it in your widget
-class CounterWidget extends WatchingWidget {
-  @override
-  Widget build(BuildContext context) {
-    // This one line makes it reactive!
-    final count = watchValue((CounterManager m) => m.count);
-
-    return Scaffold(
-      body: Center(
-        child: Text('Count: $count', style: TextStyle(fontSize: 48)),
-      ),
-      floatingActionButton: FloatingActionButton(
-        onPressed: () => di<CounterManager>().increment(),
-        child: Icon(Icons.add),
-      ),
-    );
-  }
-}
-```
+<<< @/../code_samples/lib/watch_it/counter_simple_example.dart#example
 
 That's it! When you press the button, the widget automatically rebuilds with the new count.
 
@@ -83,16 +51,7 @@ The widget automatically subscribes to changes when it builds and cleans up when
 
 Already have an app? Just add a mixin to your existing widgets:
 
-```dart
-class MyWidget extends StatelessWidget with WatchItMixin {
-  const MyWidget({super.key});
-
-  Widget build(BuildContext context) {
-    final data = watchValue((Manager m) => m.data);
-    return Text('$data');
-  }
-}
-```
+<<< @/../code_samples/lib/watch_it/mixin_simple_example.dart#example
 
 No need to change your widget hierarchy - just add `with WatchItMixin` and start using watch functions.