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