Add docs for chrome.wallpaper API

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

 {
   "alarms": {
     "Learn More": [
       {
         "link": "event_pages.html",
         "text": "Event Pages"
       }
     ]
   },
   "app_runtime": {
@@ skipping 252 matching lines @@
     ]
   },
   "usb": {
     "Learn More": [
       {
         "link": "app_hardware.html",
         "text": "Accessing Hardware Devices"
       }
     ]
   },
+  "wallpaper": {
+    "Permissions": [
+      {
+        "class": "code",
+        "text": "\"wallpaper\""
+      },
+      {
+        "link": "declare_permissions.html#host-permissions",
+        "text": "host permissions"
+      }
+    ]
+  },
   "webRequest": {
     "Permissions": [
       {
         "class": "code",
         "text": "\"webRequest\""
       },
       {
         "link": "declare_permissions.html#host-permissions",
         "text": "host permissions"
       }
     ]
   },
   "webstore": {
     "Learn More": [
       {
         "link": "https://developers.google.com/chrome/web-store/docs/inline_installation",
         "text": "Using Inline Installation"
       }
     ]
   },
   "windows": {
     "Permissions": [
       {
         "partial": "intro_tables/windows_permissions.html"
       }
     ]
   }
 }