BUG: GroupBy.idxmax/idxmin with EA dtypes
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@jorisvandenbossche thoughts on how/where to handle the skipna kwarg?
> thoughts on how/where to handle the skipna kwarg?

We don't yet handle that -> https://github.com/pandas-dev/pandas/issues/35178, https://github.com/pandas-dev/pandas/issues/33941, https://github.com/pandas-dev/pandas/issues/33942. 
Also https://github.com/pandas-dev/pandas/pull/37924 fixing idxmin/max for Series with EAs is not yet tackling it. 

So I think this is something we first need to decide upon / fix for the Series case, before doing this for groupby.

 To be clear, the open issues are mainly about the behaviour for argmin/max with skipna=False, not for idxmin/max (but since idxmin/max is typically implemented based on argmin/max, it's still impacted) @jorisvandenbossche do the blockers you mentioned have some momentum to them?  i.e. if I want to get this fixed, do i need to go work on those directly or can i just be patient for a bit? gentle ping @jorisvandenbossche thoughts on how to make this actionable? By looking into those linked issues, and trying to move them forward ..

Now, I think this PR can already fix the non-skipna case in the meantime? > Add a whatsnew note?

updated updated + green updated docstrings, not sure about testing for the NotImplementedError cases since that isnt actually desired behavior > updated docstrings, not sure about testing for the NotImplementedError cases since that isnt actually desired behavior

sure but we want to lock it down to make sure that it is not actually changed w/o doing anything about it. test added + greenish