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/context_menu_page.dart

@@ -0,0 +1,42 @@
+import 'package:flutter/cupertino.dart';
+
+class ContextMenuPage extends StatelessWidget {
+  const ContextMenuPage({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoPageScaffold(
+      navigationBar: const CupertinoNavigationBar(middle: Text('Context Menu')),
+      child: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: [
+            Text('Long press to activate context menu:'),
+            SizedBox(height: 16),
+            SizedBox(
+              width: 100,
+              height: 100,
+              child: CupertinoContextMenu(
+                actions: <Widget>[
+                  CupertinoContextMenuAction(
+                    child: const Text('Action one'),
+                    onPressed: () {
+                      Navigator.pop(context);
+                    },
+                  ),
+                  CupertinoContextMenuAction(
+                    child: const Text('Action two'),
+                    onPressed: () {
+                      Navigator.pop(context);
+                    },
+                  ),
+                ],
+                child: Container(color: CupertinoColors.activeBlue),
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}