DOC Fixing EX01 - added examples
Towards #37875

I couldn't locally build the html files for `Series.SeriesGroupBy.is_monotonic_increasing` and `Series.SeriesGroupBy.is_monotonic_decreasing`. Followed https://github.com/noatamir/pyladies-berlin-sprints/issues/13 to split the docstrings from Series - but I'm not sure if adding #GH13 to the docstring is correct

Couldn't use a variable like {klass} for `item()` to avoid hardcoding `Series` or `Index`.
I can't tell (for sure) if failing tests are related to my changes  Thanks @MarcoGorelli! - love the cat  