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 I understand they were outdated, but they were better than nothing.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 It's fine to warn they are severely outdated, but there is no reason to make them completely unavailable in other languages.

首先阅读链接的 Suspense RFC，以及 issue 中引用的已移除 Concurrent Mode 文档，然后检查主文档如何链接到它们，以及重定向在哪里配置。确定是否可以将 RFC 改编为主仓库中可翻译的文档，以及恢复旧页面是否仍然合适；当约定的介绍和重定向行为可供翻译人员使用时，即视为完成。