Add initial sphinx configuration

Add initial sphinx configuration

This is essentially the output of 'sphinx-quickstart' but with the
following changes:

- Parts of the generated Makefile are merged into the existing
  Documentation/automake.mk Makefile
- A license is added to the index.rst file
- The OVS logo is added
- A 'contents' page is added, so we don't need to include a TOC on the
  home page
- The theme is switched to 'bizstyle', which makes better use of
  horizontal real estate than the default 'alabaster' theme

Copyright is assigned to "The Open vSwitch Development Community".

Signed-off-by: Stephen Finucane <stephen@that.guru>
Signed-off-by: Ben Pfaff <blp@ovn.org>