change documentation theme
Documentation theme is changed to ReadTheDocs theme. TOC and CSS were
adjusted to fit nicely into the theme.
Version parsing is now done automatically and we no longer need to
update libtaskotron version in docs' conf.py anymore.
Sphinx-build now uses python from virtualenv. Use better approach to add
stylesheet (the old one fails with newer sphinx).
Differential Revision: https://phab.qa.fedoraproject.org/D1198