Move openstackdocstheme to extensions in api-ref
Move openstackdocstheme to extensions in api-ref

Move openstackdocstheme to extensions. According to the guide below:
https://docs.openstack.org/openstackdocstheme/latest/

Change-Id: I7e322de0357efd4df4e223346130c5c2f525a9c7

openstack/senlin,openstack/senlin,stackforge/senlin,openstack/senlin,stackforge/senlin