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.

Empieza por localizar la página principal del sitio de documentación y los puntos de entrada a los tutoriales; después, revisa cómo se presentan allí los avisos o los enlaces externos. Se considera terminado cuando el enlace solicitado a React with Hooks y el aviso explicativo aparecen en la página principal y en los tutoriales, sin esperar a la reescritura de la documentación.