docs: Add wrapper div to react's createShadowRootUi example

Related to #456.

Author: aklinker1

docs/guide/content-script-ui.md

@@ -253,8 +253,12 @@ export default defineContentScript({
       name: 'example-ui',
       position: 'inline',
       onMount: (container) => {
+        // Container is a body, and React warns when creating a root on the body, so create a wrapper div
+        const app = document.createElement('div');
+        container.append(app);
+
         // Create a root on the UI container and render a component
-        const root = ReactDOM.createRoot(container);
+        const root = ReactDOM.createRoot(app);
         root.render(<App />);
         return root;
       },