* remove local toc sidebar in non-index documents with a toctree
* change toctree rendering to only have links on leaf nodes (don't link
to intermediate toctree'd documents)
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