Add CITATION.cff template to developer pages

[matthewfeickert]

IMO a guide should be added to scikit-hep.org, probably under developer. Maybe could be combined in an "extras" page that covers license, badges, etc?

Originally posted by @henryiii in scikit-hep/hist#416 (comment)

[eduardo-rodrigues]

Hello @matthewfeickert, I know you're very busy ... but may I ask whether you reckon you will be able to write a little page for this?

I do agree that we should simply have a link as for other pages, and the actual content would be under https://github.com/scientific-python/cookie/tree/main/docs/pages/guides.

[eduardo-rodrigues]

Hello @matthewfeickert, @henryiii, is this still relevant or we can rather close it?