website: Add v21.0.0.1 Doxygen link

Change-Id: Iacd3c7738ae170ebf84e860fc1f20ee67c89af1d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/60669
Tested-by: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
1 file changed