Add full react component to docs

docs/source/developer/virtualdom.create.tsx

@@ -3,4 +3,8 @@ import * as React from 'react';
 import { Widget } from '@phosphor/widgets';
 import { ReactWidget } from '@jupyterlab/apputils';
 
-const myWidget: Widget = ReactWidget.create(<div>My Widget</div>);
+function MyComponent() {
+  return <div>My Widget</div>;
+}
+
+const myWidget: Widget = ReactWidget.create(<MyComponent />);

docs/source/developer/virtualdom.reactwidget.tsx

@@ -3,9 +3,12 @@ import * as React from 'react';
 import { Widget } from '@phosphor/widgets';
 import { ReactWidget } from '@jupyterlab/apputils';
 
+function MyComponent() {
+  return <div>My Widget</div>;
+}
 class MyWidget extends ReactWidget {
   render() {
-    return <div>My Widget</div>;
+    return <MyComponent />;
   }
 }
 const myWidget: Widget = new MyWidget();