Documentation: code style

[KulaginVladimir]

The FESTIM code is formatted with Black.

I think it'd be beneficial to explicitly specify this in the Contributing to the code section of developer's guide.

[RemDelaporteMathurin]

It would also help to show a few commands on how to format a script with the black command

black my_script.py

or

black .

[XinShen-CHN]

I found it that the content of the index.rst under devguide is different with the content of the merged ones in Developer's guide in FESTIM documentation when I want to make the modification to specify the code style.

Here's the content of the index.rst under devguide:

1) `Fork the repository <https://github.com/festim-dev/FESTIM/fork>`_

By forking the repository, you create a copy where you can safely make changes.

2) Make your changes
3) `Open a PR <https://github.com/festim-dev/FESTIM/compare>`_
4) Wait for a :ref:`maintainer<Maintainers>` to review your PR

Before merging your changes, they have to be reviewed. We ensure the changes don't break anything during the review and eventually propose/request improvements.
The time before the review will depend on the maintainers' availability.

5) When everything is in order, the maintainers will merge your PR!

Here is the content of the merged in Developer's guide in FESTIM documentation: https://festim.readthedocs.io/en/latest/devguide/index.html.

[RemDelaporteMathurin]

This is the difference between the latest and stable versions of the documentation.

stable is the latest released festim version (here 1.3.1)
latest is the latest pushed version (on the main branch)