feat: migrate React.dev to the App Router

[feedthejim]

What

This PR converts all of the react.dev codebase to use the App Router.

Why

I wanted to prepare it for dogfooding future features of React and Next.js.

In this PR

• implemented/fixed HMR when editing mdx files
• rewrote the post-processing we did to be actual remark plugins
• rewrote the RSS script to use static route handlers
• rewrote the page rendering code to use server components, streamlining some of the logic
• leveraged Next.js's metadata methods
• enabled the new Next.js Error Overlay
• added myself as a site contributor
• added a few minor polishes for perf, namely some font optimisations

Not in this PR

• leveraging parallel routes for the sidebars
• moving more content to be server components
• a deeper refactor of the code to actually use pathname normally
• a rewrite of the TOC, whose code is kinda weird today

Todo

• convert main page
• re-do MDX generation
• convert error page
• convert not found
• convert errors/[error]
• add back languages
• optional: split sidebars with parallel routes
• fix metadata gen
• add /errors back
• see if I can hack some router events alternative
• fix TOC generation
• re-do RSS generation
• double check analytics logic
• fix font flashing
• make sure the path logic transformation is actually valid
• check that the router methods have been replaced correctly

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
19-react-dev	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Feb 1, 2025 4:35pm
react-dev	✅ Ready (Inspect)	Visit Preview		Feb 1, 2025 4:35pm

[github-actions]

Size changes

📦 Next.js Bundle Analysis for react-dev

This analysis was generated by the Next.js Bundle Analysis action. 🤖

🎉 Global Bundle Size Decreased

Page	Size (compressed)
global	91.93 KB (🟢 -18.5 KB)

Details

The global bundle is the javascript bundle that loads alongside every page. It is in its own category because its impact is much higher - an increase to its size means that every page on your website loads slower, and a decrease means every page loads faster.

Any third party scripts you have added directly to your app using the <script> tag are not accounted for in this analysis

If you want further insight into what is behind the changes, give @next/bundle-analyzer a try!

One Page Changed Size

The following page changed size from the code in this PR compared to its base branch:

Page	Size (compressed)	First Load
/_error	192 B (🟡 +2 B)	92.12 KB

Details

Only the gzipped size is provided here based on an expert tip.

First Load is the size of the global bundle plus the bundle for the individual page. If a user were to show up to your website and land on a given page, the first load size represents the amount of javascript that user would need to download. If next/link is used, subsequent page loads would only need to download that page's bundle (the number in the "Size" column), since the global bundle has already been downloaded.

Any third party scripts you have added directly to your app using the <script> tag are not accounted for in this analysis

Next to the size is how much the size has increased or decreased compared with the base branch of this PR. If this percentage has increased by 10% or more, there will be a red status indicator applied, indicating that special attention should be given to this.

[rickhanlonii]

Not a server component?

[sophiebits]

error page must be a client component https://nextjs.org/docs/app/api-reference/file-conventions/error

[rickhanlonii]

sick thanks!