DOC: Remove doc/source/groupby.rst from flake8-rst exclude section in setup.cfg (pandas-dev#24178)

Author: LJArendse

setup.cfg

@@ -79,7 +79,6 @@ exclude =
     doc/source/basics.rst
     doc/source/contributing_docstring.rst
     doc/source/enhancingperf.rst
-    doc/source/groupby.rst
     doc/source/indexing.rst
     doc/source/missing_data.rst
     doc/source/options.rst