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: Ie4c3d26404a22791a9cbd2e6f14f822e7e5a6d13

openstack/magnum,openstack/magnum,ArchiFleKs/magnum,ArchiFleKs/magnum