Improve automatic citation by CITATION.cff

[glatterf42]

Last year, we noticed that the automatically-created citations (via Cite this repository generated from CITATION.cff) do not include all information that we provide in the citation file. I've opened a ticket with GitHub and from there, I found the following:

• They confirmed that our file is valid
• They noted that the automatic citation does include the doi e.g. for the citation-file-format repo

Comparing to this repo, I noticed that

• We don't provide an explicit doi key in our citation file (only as a subkey to identifiers), while the other repo does. Providing one might include this information in both available citation styles.
• Not all information is included in the citation entries. For example, references and keywords do not show up for citation-file-format either. I did ask in the ticket if these entries could be expanded, but the support answer did not say anything with regards to that.
• We provide repository-code as well as url and interestingly, the value from repository-code is included in the url keyword of the BibTex entry. This hints at the keys and their effect not being properly understood when the file was created and it won't hurt to quickly go over the GitHub docs for citation files and the citation file format docs to check we are not missing any keys we might want included. For example, we could include a version key like version: 3.8.0 since from the comparison we know this information will be included in the citations (and might be helpful; others we know of are year and month from date-released: "2021-08-09"). If we do something like this, we should update the release procedure document as well to include updating the citation file.