Document how to work with sphinxcontrib-bibtex

[lilyminium]

I think appropriate guidelines would be:

• for an internal module where you think a table may get rendered through autosummary, use footbibliography, the directive for local bibs
• for an overall references page for the entire module: use bibliography. The issue in Incompatible with sphinxcontrib-bibtex? #25 was that a table got incorrectly rendered after the correctly rendered bibliography, so as long as we take care not to use global references in a module, I think we will avoid it in the future.
  For reference, the user guide has a global bibliography: https://userguide.mdanalysis.org/stable/references.html

Also we should pin to bibtex>2.0 where the footbib was first introduced

[ianmkenney]

@lilyminium, see MDAnalysis/mdaencore#17, most likely relevant