RFE: Allow "main" for unversioned URLs

Recently, the Fedora Council attempted to change our default branch for our documentation to "main". However, we ran up against the fact that "master" is the only reserved word for unversioned URLs. Given that GitHub has changed the default branch for new repos, this is an issue that more users will encounter over time.

Ideally, we'd have a mechanism to allow multiple versions that produce unversioned URLs, or at least have it be settable on a per-repo basis. Right now, docs.fedoraproject.org builds from ~20 independent repos, so having to make a coordinated cutover would be challenging, but if that's the only workable solution, we can make it happen.