Improve experience when reaching URL of offline version

As a follow-up to #223 (closed)

Problem

Today, when visiting a URL containing an archived docs version, a user is redirected to the Archives page. A banner says to use the "Choose version" menu above, or Archives below. The "Choose version" menu links to other copies of the /archives page, rather than the original topic they intended to reach.

Proposal

Given that a user is most likely to want to view the same URL, except on a newer, online version, update the experience to:

• Keep the user on the 'bad' URL instead of redirecting to Archives.
• Offer an alternate 404 message about using the versions menu above or accessing the archive (and link to the archive page).

/labels team:engineering ~P2 versions