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