DOC: Include a small preview of the groupby() user guide in its API reference (#43880)

Author: tobiasmcnulty

pandas/core/shared_docs.py

@@ -145,7 +145,9 @@
 Notes
 -----
 See the `user guide
-<https://pandas.pydata.org/pandas-docs/stable/groupby.html>`__ for more.
+<https://pandas.pydata.org/pandas-docs/stable/groupby.html>`__ for more
+detailed usage and examples, including splitting an object into groups,
+iterating through groups, selecting a group, aggregation, and more.
 """
 
 _shared_docs[