Synnefo

docs: Update index page & Astakos related docs

• Update index page
• Rewrite the Astakos description in astakos.rst
• Move useful info from astakos.rst to the Admin Guide
• Remove obsolete info from astakos.rst
• Refactor the Admin Guide's Astakos section

docs: Update cyclades.rst and admin guide

Update archipelago-architecture image.

Update docs theme to match synnefo.org

Update the Admin Guide

Update the general Admin Guide with a new skeleton.
Moved the old content in the new sections, so that it can be
reviewed and adjusted accordingly.

Also added the Synnefo General Architecture graph in the index
page and the Admin Guide.