Automatically remove flakehell dependencies from `mkdocs_template_rdeville` as
flakehell seems to be dying.
Replace flakehell by flake9, fork of flake8 with pyproject support.
Update files accordingly.

Fix pydocstyle warning in plugins.py files.

Upgrade CI from mkdocs_template_rdeville.

Due to error when using `tox` about unused args, remove those args from
`set_nav` method in `plugins.py` file.

Add support of extra `nav` key which can be defined in
`docs/_data/extra.yml` allowing to overload the `nav` key of
`mkdocs.yml` file, allowing to forked repo to have their own `nav`
without modifying the `nav` key of the `mkdocs.yml` file.

Fix minor typo in file `docs/_data/plugins.py` not loading `extra.yml`
file correctly.

Due to error when running tox, adding better docstring in `plugin.py`
and update content of `plugin.py` to fix pylint error.

Update and

Huge rework of the `docs/.gitlab-ci.yml` to :

- Simplify the file describing the CI process.
- Use `rules` instead of `only/except` to be more flexible in case of future update.
- Update script `docs/_data/plugins.py` to better support external subrepo.
- Adding logo in PNG and SVG format.

See merge request rdeville.private/my_programs/mkdocs_template!2

Mainly updating `docs/.gitlab-ci.yml` and `docs/_data/plugins.py` to make the CI working.

Comment formatting test for now.

Use basic mkdocs_template to setup base mkdocs documentation

Add base templates folder with :

  - Requirements to build the documentation
  - Basic mkdocs.yml file
  - Template and schema for variables