Docs reorg

New docs built here: http://docs.ckan.org/en/docs-reorg/

As discussed at the CKAN summit, these reorganizes the docs into 8 top-level sections: user guide, sysadmin guide, maintainer's guide, API guide, extending guide, theming guide, contributing guide and changelog. It adds an overview of the sections to the docs front page, and also adds some explanation to the documentation guide:

http://docs.ckan.org/en/docs-reorg/contributing/documentation.html#how-the-docs-are-organized