Make 404 pages more useful

[schalkneethling]

Initially inspired by a comment from @Gitoffthelawn, @ddbeck opened an issue titled, "Make the 404 page useful". There are some good ideas in there that warrants a discussion to come to an agreed-upon actionable issue.

Quoting from the initial issue comment:

Summary

Some pages 404 for good, but unexplained reasons. The 404 page should help MDN readers find the content they're looking for with a prominent search box, links to mdn/archived-content, Firefox Source Docs, the Wayback Machine, or to raise an issue on mdn/content.

Making the 404 page useful

Over the years, lots of content has been removed from MDN, for many reasons including:

• It moved elsewhere (e.g., https://firefox-source-docs.mozilla.org/)
• It was archived (i.e., put in https://github.com/mdn/archived-content)
• It was deleted (e.g., irrelevant content)

Some of these removals have been supported by useful redirects. However, in many cases, there's no sensible redirect because there's no plausible replacement content on or off MDN, or the removal was simply mismanaged. In those cases, we should provide a 404 page that actually helps readers find the content they're looking for.

The 404 page could help by linking mdn/archived-content, Firefox Source Docs, the Wayback Machine, or to the form to create a new issue on mdn/content. Ideally, the last option would only apply when the document referrer is on MDN itself.

Related issues

Inspired by this comment from @Gitoffthelawn: mdn/yari#5484 (comment)