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: I8a6c3403192d1416cb66cc9e92ec827b339f1270

dtroyer/devstack,openstack-dev/devstack,openstack-dev/devstack,dtroyer/devstack,nawawi/openstack,nawawi/openstack