website: Updated Doxygen URLs

The Doxygen doxygen documentaton has been moved to Google Cloud
Buckets (from GitHub hosting). Furthermore, we will be hosting
multiple Doxygen's. One for each release, and one for the develop
branch. The website has been updated to accomodate this.

Jira: https://gem5.atlassian.net/browse/GEM5-335
Change-Id: I791a1ae2fe0681a08d1ebdec7a520c0b1fb45058
diff --git a/_data/documentation.yml b/_data/documentation.yml
index aa37493..c2f0f6d 100644
--- a/_data/documentation.yml
+++ b/_data/documentation.yml
@@ -19,6 +19,14 @@
         - page: Building EXTRAS
           url: /documentation/general_docs/building/EXTRAS
 
+    - title: Doxygen
+      id: doxygen-docs
+      subitems:
+        - page: Develop Branch
+          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
+
     - title: Full System
       id: fullsystem
       subitems:
@@ -225,7 +233,7 @@
     items:
        - title: gem5 Doxygen
          id: doxygen
-         url: https://gem5.github.io/gem5-doxygen
+         url: http://doxygen.gem5.org/release/current/index.html
 
   - title: Reporting Problems
     items: