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>
1 file changed