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

### Include .py file example

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

```rst
.. _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.
```