Docs: Set up redirects for old urls

[Krinkle]

Fixes #1289.

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Follows proof-of-concept from #1291 which worked. https://api.qunitjs.com/QUnit/stop now redirects to http://api.qunitjs.com/assert/async.

[platinumazure]

LGTM in general.

Wondering if we could enable some plugin to strip or add a trailing slash for all incoming requests? If so, we could maybe remove half of the redirect listings. Of course, that's beyond the scope of this PR.

[Krinkle]

@platinumazure Yeah, good idea.

I'll land this without it for now, though. In particular because it's a limited set of known urls and their variants, particularly because the old site supported the variant.

However, for the current urls there's a less strong need for it, given the variant with or without slash never worked for those urls, which means they shouldn't be reachable through normal link discovery. However, to be friendly to url-hackery and to tolerate broken tooling that may accidentally remove or add a slash, I'd certainly support making it work.