Allow adding additional documentation that is added to the auto-generated `.py` file documentation.

[a-t-0]

Include .py file example

To add a file in src/jsonmodipy/helper.py you
create a docs/source/some_name.rst file with content:

.. _helper-module:

Helper Module
===============

.. automodule:: jsonmodipy.helper
  :members:
  :undoc-members:
  :show-inheritance:

And here you can just type additional text that will be displayed on the site.