website: Add v21.0.1.0 Doxygen link

Change-Id: I0a7e4aca6100179b7a4831c8412e55339f9274e3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/47139
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