Move Suspense introduction to the main docs repo

I asked for this before, but the official docs should not lightly link to external articles to explain basic React concepts, because external articles cannot be translated.

Currently, this external RFC, available only in English, is somehow treated as the "official get-started guide" for Suspense, useTransition, etc. It is linked from many pages in the main docs, including recent blog articles. On the other hand, information in the current docs is limited, and they do not explain a Suspense forms a "catch block (boundary)", for example. It's hard for non-English speakers to understand how to use the new concurrent features now.

The React team even deleted the old Concurrent Mode docs without providing alternative articles, which means the most comprehensible introduction for Suspense is now lost at least in my language. I understand they were outdated, but they were better than nothing. The removed files are redirected to 17.reactjs.org, but 17. is not available in other languages.

Please consider the following:

1. Copy the contents of the RFC to somewhere in the main docs so that translators can work on them.
2. Meanwhile, restore the Concurrent Mode docs and revert the redirects so that non-English speakers can read them again. It's fine to warn they are severely outdated, but there is no reason to make them completely unavailable in other languages.

Beginne damit, den verlinkten Suspense RFC und die entfernte Dokumentation zu Concurrent Mode zu lesen, auf die im Issue verwiesen wird. Untersuche anschließend, wie die Hauptdokumentation auf sie verweist und wo die Weiterleitungen konfiguriert sind. Ermittle, ob der RFC in übersetzbare Dokumentation im Haupt-Repository umgearbeitet werden kann und ob die Wiederherstellung der alten Seiten noch angemessen ist; als abgeschlossen gilt die Aufgabe, wenn die vereinbarte Einleitung und das Weiterleitungsverhalten für Übersetzer verfügbar sind.