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: I7c23cc33007316c3316b3307f5f0500b244ca035

openstack/zaqar,openstack/zaqar,openstack/zaqar,openstack/zaqar