Push documentation to RTD

[icemac]

Documentation is easier accessible when it can be read on readthedocs.io.

[dataflake]

FYI, this will/should require creating a toplevel docs folder with a Sphinx configuration and moving/converting the old .stx files from the product folder into it.

[dataflake]

This is now mostly done, see http://productspluggableauthservice.readthedocs.io/en/latest/

The only thing that doesn't work is the automatic API documentation for interface files with repoze.sphinx.autointerface.