Add ability to attach citations in docstrings and markdown pages

[patcon]

scanpy is very rigourous in that many of it's methods and recipes cite scholarly sources. For example:
https://scanpy.readthedocs.io/en/stable/api/preprocessing.html

It would nice to try to do the same, to set ourselves off on the right foot.

The documentation tool we're using (mkdocs and mkdocs-material) doesn't have as much selection for extensions to help with citations. The only available plugin is no longer maintained, but I gave it a shot anyhow:
https://github.com/shyamd/mkdocs-bibtex

What this does so far:

• enables footnotes and footnotes tooltips
• added some basic bibtex citations in a docs/references.bib file
• add an example inline citation on the scanpy API page
• allows a bibliography to be embedded at the bottom of every page using footnotes (required for JS to work)
• add custom css to hide the bibliography on most every page, so only the tooltip displays (the footnotes are required). the only page the footnotes aren't hidden on is the references.md page
• add the full bibliography page in the nav