website: Add v21.1.0.1 Doxygen link

Change-Id: I0604372b84c3bbff3c5db37e29fd1a9ca5319b05
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/50047
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