Added documentation for In-App Payments API.

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

 [
   {
     "title": "Getting Started",
     "items": [
       {
         "title": "What Are Packaged Apps?",
         "href": "/apps/about_apps.html"
       },
       {
         "title": "Understand the Architecture",
@@ skipping 159 matching lines @@
   {
     "title": "Deploying",
     "items": [
       {
         "title": "Publish",
         "href": "/apps/publish_app.html"
       }
     ]
   },
   {
+    "title": "Google Services",
+    "items": [
+      {
+        "title": "In-App Payments",
+        "href": "/apps/in_app_payments.html"
+      }
+    ]
+  },
+  {
     "title": "Reference",
     "items": [
       {
         "title": "Manifest Files",
         "href": "/apps/manifest.html"
       },
       {
         "title": "Chrome JavaScript APIs",
         "href": "/apps/api_index.html"
       },
@@ skipping 18 matching lines @@
         "title": "FAQ",
         "href": "/apps/faq.html"
       },
       {
         "title": "Samples",
         "href": "/apps/samples.html"
       }
     ]
   }
 ]