refactor: Don't redirect json/content 404's to 404 page

[rschristian]

Netlify, via our config here, is set up to redirect all 404'ing requests to /404/index.html, which isn't unreasonable for most pages but our "guess and check" method of fetching translated content makes this a bit wasteful. In the event of a missing translation file, all we do is read the status code & discard the response body, which is the prerendered 404 page (~3kb w/ gzip):

preact-www/src/lib/content.js

Lines 16 to 18 in 4cf9283

	if (!r.ok && lang != 'en') {
	fallback = true;
	return fetch(url.replace(/content\/[^/]+\//, 'content/en/'));

We could (and perhaps should) build a content manifest to skip some of these needless fetches, in the meantime, we can return a smaller asset at the very least. Reduces data egress & is likely faster for Netlify to respond with.

[rschristian]

Page was removed from v10 in #1253, overlapped a lot with the refs page.