DOC: SeriesGroupby/DataFrameGroupBy is missing class documentaion from doc index
#### Problem description

https://pandas-docs.github.io/pandas-docs-travis/generated/pandas.Grouper.html#pandas.Grouper

There are many references to the GroupBy class in the index -- but there is no class interface documentation listed in the index.

We have, for example "Categorical (class in pandas)", but not "GroupBy (class in pandas)"
which would let us know which methods are available in GroupBy.
