Cupertino gallery app (#2715)

Resolves #2519 

Part of the larger effort to bring the samples/repo into a stable and
maintainable condition that _also_ remains useful. See: #2409

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

cupertino_gallery/lib/widgets/activity_indicator_page.dart

@@ -0,0 +1,17 @@
+import 'package:flutter/cupertino.dart';
+
+class ActivityIndicatorPage extends StatelessWidget {
+  const ActivityIndicatorPage({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const CupertinoPageScaffold(
+      navigationBar: CupertinoNavigationBar(
+        middle: Text('Activity Indicator'),
+      ),
+      child: Center(
+        child: CupertinoActivityIndicator(),
+      ),
+    );
+  }
+}