DatasetGroupBy does not implement quantile

The docs claim quantile works on grouped datasets, but that does not seem to be the case:

>>> import xarray as xr
>>> ds = xr.Dataset(data_vars={"a": ("x", list("abcd"))}, coords={"x": range(4)})
>>> ds.a.groupby(ds.x % 2 == 0).quantile
<bound method DataArrayGroupBy.quantile of DataArrayGroupBy, grouped over 'x' 
2 groups with labels False, True.>
>>> ds.groupby(ds.x % 2 == 0).quantile
AttributeError: 'DatasetGroupBy' object has no attribute 'quantile'

this was found while trying to silence the nit-picky sphinx warnings in #3516

[keewis]

the warnings are still there, but at least the quantile works (assuming the tests cover it). I’ll submit this as a PR.

[max-sixty]

Thank you!