Docs reorg

New docs built here: http://docs.ckan.org/en/docs-reorg/

As discussed at the CKAN summit, this reorganizes the docs into 8 top-level sections each with a different audience: user guide, sysadmin guide, maintainer's guide, API guide, extending guide, theming guide, contributing guide and changelog. The organization of the docs was already sort-of following this audience-based approach already, this just makes it explicit.

It also 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