Fix docs generation for cluster_resolvers

Adds "cluster_resolver_pip" as a dependancy to opensource contrib, and applies a standard `remove_undocumented` to clear extra symbols.

Docs are build from a bazel bulid, and without this change the cluster resolvers are not directly accessible in "tf.contirb.cluster_resolver" during the docs build, so they do not get documented.

PiperOrigin-RevId: 183993115