Update api-ref location
Update api-ref location

The api documentation is now published on docs.openstack.org instead
of developer.openstack.org. Update all links that are changed to the
new location.

Note that redirects will be set up as well but let's point now to the
new location.

For details, see:
http://lists.openstack.org/pipermail/openstack-discuss/2019-July/007828.html

Change-Id: I644db7a0b86142551c0660e4e2fe3a1a4f6eede5

openstack/horizon,openstack/horizon,NeCTAR-RC/horizon,openstack/horizon,NeCTAR-RC/horizon,ChameleonCloud/horizon,ChameleonCloud/horizon,ChameleonCloud/horizon,openstack/horizon,NeCTAR-RC/horizon,NeCTAR-RC/horizon,ChameleonCloud/horizon