docs: modifications to conf and Makefile

    - sets up autosummary and apidoc for automatic class documentation
io.nds.kerberos: minor bug fix
plotter.TablePlot: remove __slots__

    * FIXME: this class needs to updated
io.ligolw.segments: minor docstring change

    - TODO: this module needs better documentation
io.ligolw: minor import change in __init__.py
DataQualityFlag: documentation updates

    - minor fix in active segments property
    - updates to docstrings for most functions
    - made ListClass and EntryClass attributes private
docs: big update to segments documentation
docs: update to layout and CSS