website: Add v21.2.0.0 Doxygen link

Change-Id: I2445402bae37b7d9fb4403a48b5f5d32ca23c00f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/54724
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby Bruce <bbruce@ucdavis.edu>
1 file changed