Updated Tuskar README

A more descriptive Tuskar Overview based on the recently submitted
OpenStack Summit abstract.
Base documentation organization

* docs index page
   * links to wiki pages (until wiki pages are moved to docs)
   * TODO placeholders
   * text from updated README
   * link to demo video

(Ammended by jistr for .md -> .rst migration.)
Merge branch 'overview' into docs

Fixed line length in 'overview'

Conflicts:
	README.rst