Set minimum Sphinx version to 1.3
Switch the :formula: extlink to :formula_url:

The addition of the Salt domain conflicted with this role. Better to
rename this one to be more explicit and to save that real-estate for
future use.
Switch html_title to empty string instead of None
Add a cross-ref to the vmbuilder formula repo
Merge pull request #19145 from whiteinge/sphinx-fixups

Minor Sphinx fixups