feat: Create a GitLab Pages landing page.

Import the old rarian webpage, fix the links, fix the release order, and support the old Releases tarball paths.

I pushed first without the main restriction to deploy the changes so they can be inspected. They can be accessed at https://rarian.pages.freedesktop.org/rarian/.

Once everything looks good and this gets merged, we can switch the "real" subdomain to point to these pages.

Fixes #2 (closed)

@timsoft Since you showed interest I just made you a maintainer in the group and have been pinging you with questions 😀. I always appreciate a second set of eyes on things. If you don't have the time or interest, let me know!