Stop the docs site collecting 404s from mangled links (#3190)

* Move the help link off the end of the timeout message

The timeout message ended "...that can cause timeouts: https://seredis.dev/Timeouts)",
and link detectors in log viewers, chat and issue trackers take the closing bracket with
them. The docs site sees the result: /Timeouts) is far and away its most requested
missing page, along with /Timeouts)), /Timeouts)%22 (the message inside a quoted string)
and /Timeouts)/r/n (a log line's trailing escape, with URL parsing reading the
backslashes as slashes).

Putting the URL mid-sentence with a space after it is all it takes - every detector
stops at the space. The message is a little shorter for it too.

This only helps versions built from here on; everything already shipped keeps emitting
the old text, so the site needs a redirect regardless.

* Docs: link the rendered pages, and give the site a 404 page of its own

Five links pointed at the markdown source rather than the page: Transactions.md and
KeysValues.md from the CAS page, Profiling_v1.md/Profiling_v2.md, and Timeouts.md from
the 2.0 release notes. Those resolve - the site serves the sources alongside the built
pages - but as raw text/markdown, with no styling and no way onward.

The site was also using GitHub's stock 404 page, which offers nothing. Ours points at
the places people are usually looking for, and, since the missing page is so often a URL
pasted out of an exception message with the sentence's punctuation stuck to it, offers
the address with that punctuation stripped. It suggests rather than redirects, so a bad
guess costs nothing; the strip is idempotent, so there is no loop.

Also made the bare release-notes URL in the readme an explicit link, for the same reason
the timeout message no longer ends in one.