Last month, GitHub switched their default git branch from master to
main. So, if GitHub is doing it for their millions and millions of
repos and users, there are few good reasons not to for this repo.

https://github.com/github/renaming

This commit changes all references of master to main in the Antora
site configuration. This commit is already pushed to a git branch,
main, on the council-docs repo. But this commit will be opened as a
Pull Request to the master branch.

Do not merge without coordinating on the fedora-docs/docs-fp-o PR.
If the builder is still configured to build onto master, well, that
obviously will not work. Please coordinate these changes simultaneously.

Closes #86. cc: @mattdm