Improvements to CKAN documentation (meta-ticket)

[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 ckan.org/trac
• Document caching system - ticket:841
• Document config better
• Howto guide for writing an extension

Miscellaneous

• 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 ...)