(WIP) Refactor: migrate Error Decoder to App Router

[SukkaW]

cc @sophiebits @gaearon @feedthejim @rickhanlonii

Another attempt on #7437

The following things are missing:

• Hasn't fully migrated the original <Seo /> component to the new Next.js App Router and Metadata
  • The previous attempt doesn't actually fully replicate the <Seo />'s behavior: https://github.com/feedthejim/react.dev/blob/feedthejim/app-router-migration/src/utils/generateMetadata.ts
  • Mostly, the alternates metadata referencing other translation sites are missing.
  • Needs help from the Next.js Team about how metadata and URL are resolved, cc @feedthejim
    • Ideally, the ./ convention should be used and let Next.js automatically generate the URL
• Not sure if usePathname() can safely replace useRouter().asPath.split here, needs more review and testing.
  • Also needs confirmation from Next.js Team here.
• React Error Codes fetching still uses unstable_cache from next/cache, because I really couldn't find a better alternative for caching the fetched React error codes across pages/requests cc @feedthejim

TBD in the future PRs:

• Migrate MDX infra from the current approach (revive everything in the client component) to use a server-first approach.