Document adding citations in BibTex format

[ojeda-e]

Is your feature request related to a problem? Please describe. By introducing references in the BibTex format (See https://github.com/MDAnalysis/mdanalysis/pull/3663), citations in reStructuredText are no longer accepted. It would be useful for current or upcoming contributors to have clear instructions on how to add new references to docs using the BibTex Sphinx extension.

Describe the solution you'd like Add a section describing how to add BibTex references/citations to MDAnalysis docs.

Describe alternatives you've considered (open to discussion)

• [ ] Create a new section titled "Adding new references in BibTex format" or something similar.
• [ ] Add a paragraph in https://userguide.mdanalysis.org/stable/contributing_code.html#working-with-the-code-documentation with a link to the section above. By adding this link in the first paragraphs the reader can easily jump to the content.

Additional context Issue opened as suggested by @orbeckst in https://github.com/MDAnalysis/mdanalysis/pull/3663#pullrequestreview-1047256196

[jaclark5]

To compile the docs, pyyaml > 3.01, should be added to installation of the package.