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/popup_surface_page.dart

@@ -0,0 +1,36 @@
+import 'package:flutter/cupertino.dart';
+
+class PopupSurfacePage extends StatelessWidget {
+  const PopupSurfacePage({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoPageScaffold(
+      navigationBar: const CupertinoNavigationBar(
+        middle: Text('Popup Surface'),
+      ),
+      child: Center(
+        child: CupertinoButton(
+          child: const Text('Show Popup Surface'),
+          onPressed: () {
+            showCupertinoModalPopup<void>(
+              context: context,
+              builder: (BuildContext context) {
+                return CupertinoPopupSurface(
+                  child: Container(
+                    color: CupertinoColors.white,
+                    width: 200,
+                    height: 200,
+                    child: const Center(
+                      child: Text('This is a popup surface.'),
+                    ),
+                  ),
+                );
+              },
+            );
+          },
+        ),
+      ),
+    );
+  }
+}