docs: apply some redirects

docs: apply some redirects

Here's a few redirects that we should apply, in case the redirects
gets removed in the future.

These are mostly of the 301 (moved permanently) kind, but also a
few where the site probably *should* have used the permanent
error-code.

Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19574>