website: Added gem5 20.0.0.0 Doxygen

Change-Id: Ibf372eac023c49a93ab4c48f9046062cfef09fc2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/29632
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 3f7ee46..38a7aae 100644
--- a/_data/documentation.yml
+++ b/_data/documentation.yml
@@ -26,6 +26,8 @@
           url: http://doxygen.gem5.org/develop/index.html
         - page: v19.0.0.0
           url: http://doxygen.gem5.org/release/v19-0-0-0/index.html
+        - page: v20.0.0.0
+          url: http://doxygen.gem5.org/release/v20-0-0-0/index.html
 
     - title: gem5 Resources
       id: gem5_resources