Add documentation guidelines for writing docs

New documentation guidelines built here: <http://docs.ckan.org/en/942-documentation-guidelines/documentation-guidelines.html>, this pull request also adds a section at the top of the front page: <http://docs.ckan.org/en/942-documentation-guidelines/index.html>.

Similar to the coding standards we have for python etc. but for the docs (and less strict).

This is branched off <https://github.com/okfn/ckan/pull/769>, after <https://github.com/okfn/ckan/pull/769> is merged the pull request diff should correct itself.

Before we merge this I should probably edit the existing docs to follow these guidelines at least in the most obvious ways (eg. section title capitalization).