doc: replace the current documentation theme with RTD
    
    Our current theme requires a lot of configuration and the result
    doesn't look very pleasant.
    
    The left panel navigation is just annoying because once you are in
    some section (e.g. User Documentation), it doesn't allow you to
    quickly jump to a different one of a same level (e.g. Maintenance
    Documentation).
    
    Also, the warning and note boxes doesn't render properly.
    
    I suggest to use RTD theme, we use it also for python-copr
    package documentation.