website: Add Doxygen link for v21.1 Change-Id: I4eccfad942485374272abe21b93d653ed3d1ddfd Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/48743 Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu> Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu> Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>