Pretty much completely rewritten theme with custom HTML translator and a
few parts of the old theme extracted to their own extensions.
Banner images thought not to be that huge after all, and not worth the
hassle of them living in a different repository.
co-authored with @stefanorigano
* rename functional -> business
* fix navbarification of main toctree: would stop processing after it
hit the first toc item without children, and the business links (with
children) follow the web service toc item (without children)