doc/Makefile: run sphinx in serial mode
    
    Unfortunately, using pydata_sphinx_theme extension generates warnings in
    sphix processing. These warnings cause documentation build to be
    considered a failure:
    
    WARNING: the pydata_sphinx_theme extension is not safe for parallel writing
    WARNING: doing serial write
    ....
    build finished with problems, 2 warnings.
    make: *** [Makefile:24: html] Error 1
    
    Since the build is already doing a serial write, enforce it from start.
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Francisco Trivino <ftrivino@redhat.com>