docs: Redirect outdated RTD docs to current GH pages

[janbrasna]

There are still docs built previously from now-archived repos available on readthedocs.io without a notice they're not being up to date (with the exception being autopush, where banner at the top warns about the fact, but people arriving to an inpage link won't be able to see that anyways).

ReadTheDocs allows setting a "Force Redirect" even for published pages (might need contacting their support for that though), so you don't have to change the underlying md docs in archived repos, just redirect away everything from the published rtd site.

It would be worth doing if someone still has admin access to the rtd projects:

• https://mozilla-push-service.readthedocs.io/ [*] → https://mozilla-services.github.io/autopush-rs/
• https://autopush.readthedocs.io/ [*] → https://mozilla-services.github.io/autopush-rs/

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