Add Doxygen link for v21.0.0.0

Change-Id: Id9c11f1e6d6ec00129d729492c531e00a095b33e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/43525
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
diff --git a/_data/documentation.yml b/_data/documentation.yml
index 4a2acaf..02a6494 100755
--- a/_data/documentation.yml
+++ b/_data/documentation.yml
@@ -38,6 +38,8 @@
           url: http://doxygen.gem5.org/release/v20-1-0-1/index.html
         - page: v20.1.0.5
           url: http://doxygen.gem5.org/release/v20-1-0-5/index.html
+        - page: v21.0.0.0
+          url: http://doxygen.gem5.org/release/v21-0-0-0/index.html
 
     - title: gem5 Resources
       id: gem5_resources