docs: only link to old docs from html

docs: only link to old docs from html

Only the HTML builder knows about these files, so we can't really link
to them when using other builders. This only matter to the link-checker
right now, because that's the only other builder we use.

It's a bit sad that we can't linkcheck files like these, but it doesn't
work either way. Another way of making it work could be to use the
download-role for these files. I'm not sure I think that's any better,
as that makes the code read a bit more confusingly to me; the intent
isn't to *download* the files, but to view them as raw-text.

I could go either way here, though. Neither is fantastic IMO, but
neither is a disaster either.

Reviewed-by: Corentin Noël <corentin.noel@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25585>