Use case: update data registry or combine with model registry

[dberenbaum]

Now that we have a model registry and have even expanded it to work for non-model artifacts, we should revisit the data registry use case. I think this should recommend using Studio to act as the registry across projects rather than have a dedicated data registry repo (see this comment for why I think it's an improvement over the current approach in the use case).

We also should think about how to position this use case. It gets more hits than the model registry, so we should revisit how to explain it alongside the model registry. We may also have some broader data registry use case that involves DVCX and want to redirect there instead. No matter what we decide, we should at least stop recommending the approach of using a centralized Git repo as a data registry.