[Beta] Use Server Components

This is not actually feasible yet but it would be nice to prepare so that when they're stable-ish in Next, we can use them.

I started a branch in https://github.com/reactjs/reactjs.org/tree/sc which only has http://localhost:3000/apis/usestate and http://localhost:3000/learn/props stubs which I use for testing.

I've ran into a few things already:

• Basic stub
  • Need a layout system (I hacked one in)
  • Need to have a way to pass components to MDX
  • Minor bug I worked around https://github.com/vercel/next.js/issues/34595
• Shared components can't import client components (https://github.com/vercel/next.js/issues/34594), seems like a big problem for any serious code conversion
• Actually renaming the files (it would be nice to have a script to do that)
  • We can actually do this before adopting SC so that by the time they're out, it's easy to switch. I.e. let's try to convert the site to use .client for things that need to be client components, and to follow SC rules. And then see how far we can keep things on the server, i.e. how many files don't really need .client if we do a bit of refactoring.
• There's no support for SSG, it seems, yet. Next generates something with server runtime. Not sure when SSG is on the roadmap (if at all?)

首先檢查 sc 分支及其 /apis/usestate 和 /learn/props stub，然後查看所連結的、關於共用元件和 SSG 的 Next.js 限制。該 issue 沒有指定檔案或測試；要完成此項工作，需要就重新命名為 .client、採用 Server Components 以及支援 SSG 的範圍達成共識。