website: Add link to v21.2.1.1 Doxygen link

Change-Id: I620ed9e85aadb23a8e912841b8ed2baa56e58e05
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/59353
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby Bruce <bbruce@ucdavis.edu>
1 file changed