I don't think it's spinning, either, because the window alignment indicates other wise and, as you say, there is artificial gravity in CD. But I'm not sure that the ring is there for docking purposes. That function seems to be mainly served by the big gantry structure, and it would be simpler to have more of those if you need the space.
I'm leaning towards the theory that the "tower" is the military and administrative portion of the station, and the "ring" houses the civilian population and businesses, i.e. "startown".