[super] Improvements to CKAN documentation

[rgrp]

List of improvements to CKAN documentation (result of ticket:904). Should convert most items into sub-tickets as we go along.

Major

• (Make blog.ckan.org into ckan.org and move ckan.org to trac.ckan.org/)
• Document caching system - ticket:841
• Howto guide for writing an extension - started on this at ​http://wiki.ckan.net/Extending_CKAN

Miscellaneous

• Contributors / credits page: ticket:928
• (Data proxy documentation: move it to packages.python.org/dataproxy)
• (List dataapi extensions on http://ckan.org/wiki/Extensions (Name, summary string + point to pypi page ...))
• Document license configuration and license system - ticket:973
• Refactor trac frontpage moving general feature overview to ckan.org
• Create proper roadmap information on ​http://trac.ckan.org/
• Add link to wiki.ckan.net to ckan.org

[rgrp]

Major update including notes of what has been done (where not a separate ticket) and addition of a few items.

[rgrp]

Closing this ticket as #841 is minor. More work on docs can go in new tickets.