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>
diff --git a/_data/documentation.yml b/_data/documentation.yml
index f414506..86c636d 100755
--- a/_data/documentation.yml
+++ b/_data/documentation.yml
@@ -52,6 +52,8 @@
           url: http://doxygen.gem5.org/release/v21-1-0-1/index.html
         - page: v21.1.0.2
           url: http://doxygen.gem5.org/release/v21-1-0-2/index.html
+        - page: v21.2.0.0
+          url: http://doxygen.gem5.org/release/v21-2-0-0/index.html
 
     - title: gem5 Resources
       id: gem5_resources