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/button_page.dart

@@ -0,0 +1,42 @@
+import 'package:flutter/cupertino.dart';
+
+class ButtonPage extends StatelessWidget {
+  const ButtonPage({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return CupertinoPageScaffold(
+      navigationBar: const CupertinoNavigationBar(middle: Text('Button')),
+      child: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: <Widget>[
+            Text(
+              'CupertinoButton widget',
+              style: CupertinoTheme.of(context).textTheme.textStyle,
+            ),
+            const SizedBox(height: 16),
+            CupertinoButton(child: const Text('Enabled'), onPressed: () {}),
+            const SizedBox(height: 16),
+            const CupertinoButton(onPressed: null, child: Text('Disabled')),
+            const SizedBox(height: 32),
+            Text(
+              'CupertinoButton.filled widget',
+              style: CupertinoTheme.of(context).textTheme.textStyle,
+            ),
+            const SizedBox(height: 16),
+            CupertinoButton.filled(
+              child: const Text('Enabled'),
+              onPressed: () {},
+            ),
+            const SizedBox(height: 16),
+            const CupertinoButton.filled(
+              onPressed: null,
+              child: Text('Disabled'),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}