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.

Beginne damit, die Startseite der Dokumentationswebsite und die Einstiegspunkte zu den Tutorials zu finden, und prüfe anschließend, wie Hinweise oder externe Links dort dargestellt werden. Als erledigt gilt die Aufgabe, wenn der angeforderte Link React with Hooks und der erläuternde Hinweis auf der Startseite und in den Tutorials erscheinen, ohne auf die Überarbeitung der Dokumentation zu warten.