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
* use static imports in java examples to make them terser
* inline ``domain`` in java and php example to make examples more
self-contained
* try to extend/improve Model.write's docstring
* add convenience kwarg to fields_get, mostly for user-driven
introspection
Closes#3689
* add latex support for exercise admonition
* latex freezes/crashes on {HEAVY WIDE-HEADED RIGHTWARDS ARROW} so
replace them by more manual arrows
* add some preamble configuration, may not even be necessary
* generate WS setup code only in HTML output. WS doc in latex still
isn't great as it displays all 4 languages one after the other,
ideally they should be tagged or something, so only one language at a
time is generated in non-HTML outputs