gh-37828: Fix FAQ on DOIs. Put "universal" DOI to CITATION.cff*
This explains how to get the right DOI to use to cite Sage.
Also, currently all our DOI values there and in CITATION.cff,
CITATION.cff.in point to 9.5 DOI, which is wrong.
Here we replace the latter with DOI which automatically forwards to the
latest Sage Zenodo-generated DOI.
URL: https://github.com/sagemath/sage/pull/37828
Reported by: Dima Pasechnik
Reviewer(s): Dima Pasechnik, Kwankyu Lee
