Show Manager class in comparison example

Added manager region to comparison example and included it in the documentation so readers can see where the combined 'bothPositive' observable comes from.

Author: escamoteur

code_samples/lib/watch_it/multiple_values_comparison_example.dart

@@ -3,6 +3,7 @@ import 'package:watch_it/watch_it.dart' hide di;
 import 'package:listen_it/listen_it.dart';
 import '_shared/stubs.dart';
 
+// #region manager
 class Manager {
   final value1 = ValueNotifier<int>(0);
   final value2 = ValueNotifier<int>(0);
@@ -13,6 +14,7 @@ class Manager {
     (v1, v2) => v1 > 0 && v2 > 0,
   );
 }
+// #endregion manager
 
 // #region separate_watches
 class SeparateWatchesWidget extends WatchingWidget {

docs/documentation/watch_it/watching_multiple_values.md

@@ -69,7 +69,9 @@ When you have multiple event sources of the **same type** that should trigger th
 
 ## Comparison: When to Use Each Approach
 
-Let's see both approaches side by side:
+Let's see both approaches side by side with the same Manager class:
+
+<<< @/../code_samples/lib/watch_it/multiple_values_comparison_example.dart#manager
 
 <<< @/../code_samples/lib/watch_it/multiple_values_comparison_example.dart#separate_watches