Migrate default git branch from master to main (closes #86)
    
    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
    
    Signed-off-by: Justin W. Flory (he/him) <git@jwf.io>