Move state link to developer docs

The state wiki has been moved/deprecated and the
developer docs at http://docs.openstack.org/developer
are now the better source of this information.

Update the code to follow the new and improved link.

Change-Id: If0ce7be53d7741015aa738cfd6e0c2c708a22c6d
Improve documentation for engines

Partially implements blueprint sphinx-docs.

Change-Id: I75428a42ff2a53ea50304be6a497c6a17a45db16
Merge "Move state link to developer docs"
Merge "Improve documentation for engines"