Fix missing doc section for `plotting` module

Context

Currently the module plotting/plot.py does not appear in

Does not appear in the modules list in the docs:

Expected result

When accessing the https://mutt_data.gitlab.io/muttlib/modules.html

The reference for the module plotting should appear in the docs. This includes this file:

• https://gitlab.com/mutt_data/muttlib/-/blob/master/muttlib/plotting/plot.py In particular this function create_forecast_figure