hide Index.get_values in docs and IPython tab completion
In #28621 I hid the deprecated method ``Index.set_value`` in the docs (in ``indexing.rst``) and in the REPL/Ipython (by adding it to ``Index._deprecations``). I like that as it hides not-recommended parts of the pandas API and guides users away from deprecated methods.

I've given the already deprecated method ``Indexc.get_values`` the same treatment in this PR. If there's consensus about it, I could go through various deprecations (at least the more obscure ones, like ``get_ftype_counts`` and ``get_ftype_counts`` etc.).