doc: use new zerovm Sphinx theme

This is a rough adaptation of the "nature" theme for Sphinx with the
colors used on zerovm.org. Things like fonts, text sizes, paddings and
margins are still missing.

I'll most likely repackage this later as a stand-alone Python project
so we can reuse it in other contexts.
tox: include {posargs} in sphinx-build cmdline

This is very useful for testing the documentation: you can run

  $ tox -e doc -- -E

to completely rebuild the documentation (the -E flag makes
sphinx-build ignore the cached environment).
Merge pull request #64 from mgeisler/zerovm-sphinx-theme

Add a ZeroVM Sphinx theme for docs