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

Add base templates folder with :

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