Improve author list experience of the databook

[rcpeene]

We discussed the idea of being able to record the non-commit contributions to the databook such as issues, reviews, comments etc. This would need to be done through the Github API. If possible, it would be best to include it as part of the sphinx extension.

[jeromelecoq]

We should be able to list:
_authors that contributed in various ways (reviewers, experimentalists, package contributor, nwb packager, data contributors).
_Make these visible in the author list
_allow to reorganize authors in code and while using the notebook.

[rcpeene]

• Add description for author categories
• remove "additional authors" section

[rcpeene]

When it is available for regular use, we should use mystmd.org

[rcpeene]

More specific temporary info in #371