Docserver: Add annotation to intro_tables.json to restrict content by platform

chrome/common/extensions/docs/templates/json/intro_tables.json

 {
   "alarms": {
     "Learn More": [
       {
         "link": "event_pages",
         "text": "Event Pages"
       }
     ]
   },
   "app_runtime": {
@@ skipping 196 matching lines @@
       {
         "link": "gcm_server",
         "text": "API Reference for GCM service"
       }
     ]
   },
   "runtime": {
     "Learn More": [
       {
         "link": "app_lifecycle",
-        "text": "Manage App Lifecycle"
+        "text": "Manage App Lifecycle",
+        "extension_types": ["platform_app"]
       },
       {
         "link": "event_pages",
         "text": "Event Pages"
       }
     ]
   },
   "serial": {
     "Learn More": [
       {
@@ skipping 99 matching lines @@
     ]
   },
   "windows": {
     "Permissions": [
       {
         "partial": "intro_tables/windows_permissions.html"
       }
     ]
   }
 }