website: Add a link to the v21.2.1.0 Doxygen

Change-Id: Ic816253447fc10894a0a19395041a12eb5f800ba
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/56569
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby Bruce <bbruce@ucdavis.edu>
1 file changed