Improve documentation

e.g.

* https://froide.readthedocs.org/en/latest/gettingstarted/ has empty sections at the bottom on solr and celery.
* https://froide.readthedocs.org/en/latest/models/ is empty (how about a simple diagram built in gdocs drawings ...?)

Also how about using the snazzy okf sphinx theme: https://github.com/okfn/sphinx-theme-okfn (detailed instructions at: https://github.com/okfn/sphinx-theme-okfn#how-to-use)