feat(docs-infra): add v6 to the aio version picker

Author: IgorMinar

aio/content/navigation.json

@@ -748,6 +748,7 @@
   ],
 
   "docVersions": [
+    { "title": "v6", "url": "https://v6.angular.io" },
     { "title": "v5", "url": "https://v5.angular.io" },
     { "title": "v4", "url": "https://v4.angular.io" },
     { "title": "v2", "url": "https://v2.angular.io" },