website: Add Doxygen link for v21.1.0.2 Change-Id: I9d1c08fd8859cad525b80dd2cd5193b927ea6deb Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/50768 Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu> Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu> Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>