Improve developer documentation

* Merge docs/index.rst into README.rst
* Leave only a short introduction and links to other doc pages
  in docs/index.rst
* Add a link to https://wiki.openstack.org/wiki/Tuskar/Devtest
* Fix 404 links
* Fix inconsistencies in the spelling of 'Tuskar UI'
* Fix typos

Change-Id: Iafa76efdb22043434cf3879d722d9ddcccd807d4