Update citation webpage
Follow-up of #32388, addressing #24036 

I will leave it to the pandas team to decide whether to put in there a BibTeX entry with the concept DOI or a specific version, some options of dealing with this are described [in this comment](https://github.com/sherpa/sherpa/pull/634#issuecomment-553668211).

Note how [this comment thread](https://github.com/pandas-dev/pandas/pull/32388#discussion_r386392799) on the previous PR asked to replace the author list by "The pandas development team". However, if users go to Zenodo to get the correct BibTeX entry of the version they're actually using, their citation will contain the full author list provided in Zenodo.