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.).
Does this make the doc unaccessible or just not shown in the API reference?  The removes the method name from Ipython (``Index._deprecations``) and in the doc TOC (``indexing.rst``). The underlying document is however still around, e.g. for ``Index.set_value``, see here: https://dev.pandas.io/docs/reference/api/pandas.Index.set_value.html#pandas.Index.set_value

My interest is getting these out of the ipython tab-completion, so tabbing is more focused. I could revert the removal in the doc TOC, as I can see it's reasonable that deprecated methods are shown there. Makes sense. I'm fine with this either way - @TomAugspurger or @jorisvandenbossche might have some insights into how we've historically handled deprecated functions in the TOC +1 on removing them from tab completion (did that recently as well for attributes).

For the docs, in the autosummary tables they get a clear "DEPRECATED" in front (see eg in https://pandas.pydata.org/pandas-docs/stable/reference/series.html), there has been done some work to do that automatically. So I am also fine with keeping them, as it is quite visually clear they are deprecated. But of course, if it are many, it also clutters the page. 
(and for people looking at those pages, they might actually realize that some methods they are using are deprecated) 
But no strong opinion about it. thanks . I wonder if we can have a test generally to avoid these issues? e.g. we should try to see if any tab completables show a deprecation warning and then fail the test. @jreback I answered about that in the other PR as well: https://github.com/pandas-dev/pandas/pull/28524#issuecomment-533609025 
Someone would need to find out how to write a test directly with jedi, as we already have this test for IPython and it doesn't catch those warnings. 
Also, this is only for attribtues. Methods are not executed so don't raise a warning on tab-completion, so can't be catched that way.
