9a07a459 added "override=True" to silence a Sphinx warning in about
the address node already existing, however the override=True parameter
was added in Sphinx 1.4 (alongside the warning), so this breaks in
1.2.
Only pass in override=True if we're in 1.4 or later.
Closes#18232
* add Sphinx 1.6 compatibility (use app.set_translator when
available as in that case programmatically setting
html_translator_class is broken)
* fix a bunch of code block lexers to avoid warnings & get better
coloration
* add a few options which were referred to without being actually
defined
* fix some rST formatting
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
Moisés López