Salvage 404s that are 404s because of lack of /docs/

Take https://developer.mozilla.org/en-US/DOM/element.addEventListener for example. It came up in https://github.com/mdn/content/issues/2073

What ought to happen is that the fundamental redirects should have…

/en-US/DOM/(*) --> /en-US/docs/DOM/$1

but that list is very long. As of today, it’s basically anything where the word after the locale isn’t search or settings or sign-up or whatever we come up with next.

A simple fix would at least be to make the 404 page a bit more useful. A piece of async XHR could do something like:

// pseudo-code

if (VALID_LOCALES.has(currentLocale) && location.pathname.split('/')[1] !== 'docs') {
  const tryURL = location.pathname.split('/').splice(1, 0, 'docs').join('/') + '/index.json'
  const r = await fetch(tryURL)
  if (r.ok) {
    showGoodNews(r.url)
  }
}

[peterbe]

I looked again, in Google Analytics. We have virtually no 404s that could easily be fixed with a redirect (that injects /docs/ after the locale).

[peterbe]

@escattone Can you tie your PR to this issue that adds it to the fundamental redirects?