Add docs to main nav

Author: rchinn1

src/_data/sidenav/main.yml

@@ -400,6 +400,8 @@ sections:
       title: Audiences Overview
     - path: '/engage/audiences/account-audiences'
       title: Account-level Audiences
+    - path: '/engage/audiences/generative-audiences'
+      title: Generative Audiences
     - path: '/engage/audiences/organization'
       title: Organize Audiences