Add main developer page

This adds the main ('index') page for developers (to be available
at http://docs.openstack.org/developer/python-ironicclient/).
It has minimal information, and is a place holder so that we can
start adding documentation.

This also fixes issues with 'python setup.py build_sphinx':
- was getting "error: 'source_dir' must be a directory name"
  because sphinx 1.2b3 was being installed instead of 1.1.2
  (see https://bugs.launchpad.net/openstack-ci/+bug/1259511)
- was getting "sphinx.errors.ExtensionError: Could not import
  extension oslo.sphinx (exception: No module named sphinx)"
- was complaining about missing '_theme' (oslosphinx has the theme)

Change-Id: I47ad7e10c20b80a99ae36755b2f7f67c080676fd
Partial-Bug: #1300990