Documentation work-in-progress

* added how-to section and how-to guides for local development & basic site creation
* added notes on documentation style docs/improving.rst mostly cribbed from
django CMS docs.
* new Introduction section docs/introduction.rst to be expanded over time into
separate pages.
* restructured docs (also explained in improving.rst) - main sections each with their
own style and purpose:
 * introduction
 * how-to
 * reference

Maybe hyperlink compass?