Add Doxygen link for v21.0.0.0

Change-Id: Id9c11f1e6d6ec00129d729492c531e00a095b33e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/43525
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