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/sheet_page.dart

@@ -0,0 +1,39 @@
+import 'package:flutter/cupertino.dart';
+
+class SheetPage extends StatelessWidget {
+  const SheetPage({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoPageScaffold(
+      navigationBar: const CupertinoNavigationBar(middle: Text('Sheet')),
+      child: Center(
+        child: CupertinoButton.filled(
+          child: const Text('Show Sheet'),
+          onPressed: () {
+            Navigator.of(context).push(
+              CupertinoSheetRoute<void>(
+                builder: (BuildContext context) {
+                  return CupertinoPageScaffold(
+                    navigationBar: CupertinoNavigationBar(
+                      middle: const Text('Sheet'),
+                      trailing: GestureDetector(
+                        child: const Icon(CupertinoIcons.xmark),
+                        onTap: () {
+                          Navigator.of(context).pop();
+                        },
+                      ),
+                    ),
+                    child: const Center(
+                      child: Text('This is a sheet'),
+                    ),
+                  );
+                },
+              ),
+            );
+          },
+        ),
+      ),
+    );
+  }
+}