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 I understand they were outdated, but they were better than nothing.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 It's fine to warn they are severely outdated, but there is no reason to make them completely unavailable in other languages.

まず、リンクされている Suspense RFC と、issue で参照されている削除済みの Concurrent Mode ドキュメントを読み、次にメインドキュメントからそれらへどのようにリンクされているか、またリダイレクトがどこで設定されているかを確認します。RFC をメインリポジトリで翻訳可能なドキュメントに適用できるか、また古いページを復元することが今でも適切かを判断します。合意された導入文とリダイレクトの動作が翻訳者に利用可能になれば完了です。