Revise Citation section of GH repo guide

[egrace479]

I endeavored to simplify the CITATION.cff template as follows:

• Created two "content tabs" to default to the standard template (Recommended) and include an extended option.
  • This required adding one more markdown_extension config in the mkdocs.yaml.
• Admonitions are in the content tabs to encourage the use of a .zenodo.json paired with the citation file.
• All content about the extended option is within its tab.
• Removed the validator tool link, replacing it with a note to check the formatting on the branch. That tool just checks it's valid yaml; it doesn't guarantee citation formatting.
  • This further supports @hlapp's suggestion of a "student project to stand up a single-page serverless web app that asks for the metadata and outputs the CFF file the way we want it".

I also added the "references" key to the .zenodo.json template for completeness.

Closes #64.

[hlapp]

LGTM.

One comment re: wording to perhaps consider: That we generally want to ask people to cite both software and paper (if there is a paper) is an institutional practice we chose. But prioritizing the paper citation can also be a choice, which can result in actually wanting the paper citation to show up on the sidebar on GitHub (and perhaps including the code citation BibTeX in the README instead).

I don't have a good suggestion for how to convey this being a choice when instantiating the template, and of course there are a variety of reasons why code as a first-class citizen in citation networks should be promoted. So maybe this is best deferred to considering at a later time.