Installation of documentation via sphinx, django-sphinx (sphinxdoc).

Haystack search facility was required and is installed, with Whoosh engine.

Requirements updated.

Instructions for setting things up:
haystack: http://django-haystack.readthedocs.io/en/v2.5.0/ (choice: Whoosh engine)
sphinx: http://www.sphinx-doc.org/en/1.5.1/
sphinxdoc: http://django-sphinxdoc.readthedocs.io/en/latest/

In particular, a Project has to be defined in app sphinxdoc in order for docs to be displayed (starting at /docs/).

New host_settings needed:
HAYSTACK_PATH (for the location of the Whoosh index (suggestion: project's root); make sure this is not readable by site visitors)