[Extension Docs]update to side navigation

User Interface document was moved into Apps during merge.
This update moves it under Extensions.

Bug: None
Change-Id: Ic7a3f53ac24f3b4949940eb237cb572d3420202f
Reviewed-on: https://chromium-review.googlesource.com/996237Reviewed-by: Meggin Kearney <mkearney@chromium.org>
Commit-Queue: Crystal Lambert <crystallambert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548129}

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

@@ -31,6 +31,10 @@
                 "title": "Content Scripts",
                 "href": "/extensions/content_scripts"
               },
+              {
+                "title": "Design User Interface",
+                "href": "/extensions/user_interface"
+              },
               {
                 "title": "Protect User Privacy",
                 "href": "/extensions/user_privacy"
@@ -300,18 +304,6 @@
                 "title": "What Are Chrome Apps?",
                 "href": "/apps/about_apps"
               },
-              {
-                "title": "Design User Interface",
-                "href": "/extensions/user_interface"
-              },
-              {
-                "title": "Design User Interface",
-                "href": "/extensions/user_interface"
-              },
-              {
-                "title": "Design User Interface",
-                "href": "/extensions/user_interface"
-              },
               {
                 "title": "Extension Quality Guidelines FAQ",
                 "href": "/extensions/single_purpose"