doc: Use theme and extensions from JuliaDoc
improve html doc layout on mobile devices
doc/pdf: remove python module index
doc: fix templating and restore listpkg
doc: put sidebarintro back

The links are specific to the main doc, so this really does belong here. Also
move the translation links here.
doc: put TOC depth back to 1
doc: flip some generation settings off
doc: clear up some warnings from Sphinx
doc: update Makefile and README
Merge pull request #2379 from pao/topic/2123

Move Sphinx plugins and format to Python package