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.