docs: move gallium docs to docs.mesa3d.org

Here's a follow-up to !5691 (merged), that takes the next step of moving the Gallium docs into a subdir of the main docs. This exposes the docs on docs.mesa3d.org.

In addition, there's a few cleanups, where two pre-existing gallium docs in the main docs is moved into the gallium subdir, and redirects are created to avoid breaking external URLs.