DOC Add explanation for bibtex to contributor guide

[hildeweerts]

As discussed in #1048, we have agreed that for the BibTex references:

• We encourage people to simply copy the BibTex entry from the original source (e.g., ACM/Arxiv/etc.)
• The only change that needs to be made is that the bibtex id should be in the format <author-last-name><4digit-year><keyword>, as this makes it easier to write and review texts compared to more convoluted bibtex id's.

In addition, I would suggest to point new contributors to:

• The refs.bib file
• The commands that can be used for referencing. This was already mentioned in this comment, so @iofall may already be working on this. Explaining footcite, footcite:p:, footcite:t:, and .. footbibliography:: would be sufficient.

This should be added to the "Contributing documentation" section of the contributor guide.