Add some missing voiceover labels in gallery (#22753)

examples/flutter_gallery/lib/demo/shrine/shrine_order.dart

@@ -272,6 +272,7 @@ class _OrderPageState extends State<OrderPage> {
           );
         },
         backgroundColor: const Color(0xFF16F0F0),
+        tooltip: 'Add to cart',
         child: const Icon(
           Icons.add_shopping_cart,
           color: Colors.black,

examples/flutter_gallery/lib/gallery/demo.dart

@@ -206,6 +206,7 @@ class MaterialDemoDocumentationButton extends StatelessWidget {
   Widget build(BuildContext context) {
     return IconButton(
       icon: const Icon(Icons.library_books),
+      tooltip: 'API documentation',
       onPressed: () => launch(documentationUrl, forceWebView: true)
     );
   }
@@ -226,7 +227,10 @@ class CupertinoDemoDocumentationButton extends StatelessWidget {
   Widget build(BuildContext context) {
     return CupertinoButton(
       padding: EdgeInsets.zero,
-      child: const Icon(CupertinoIcons.book),
+      child: Semantics(
+        label: 'API documentation',
+        child: const Icon(CupertinoIcons.book),
+      ),
       onPressed: () => launch(documentationUrl, forceWebView: true)
     );
   }