Add pointer to "React with Hooks" docs port until docs rewrite is ready

Per #3308 , the React team is working on a complete rewrite of the React docs, which is great.

Until then, the tutorials and examples still show classes as the default.

Meanwhile, there's a fantastic port of the current React docs that updates all examples and tutorials to show hooks as the primary approach, at https://reactwithhooks.netlify.app/ .

I'd like to suggest that we add some kind of a header notice or link on the front page and the tutorials that says something like:

We're currently working on a rewrite of the React docs to improve explanations and teach React hooks as the default approach. In the meantime, you may want to look at https://reactwithhooks.netlify.app/ to see these same tutorials written using React hooks.

まずドキュメントサイトのトップページとチュートリアルへの入口を特定し、そこにお知らせや外部リンクがどのように表示されているかを確認します。ドキュメントの書き直しを待たずに、要求された React with Hooks リンクと説明用のお知らせがトップページとチュートリアルに表示されれば完了です。