docs: Add Device Art Generator for Nexus devices to d.a.c/distribute.

Most of this code is adapted from my code.google.com/p/android-ui-utils
project.

Change-Id: If6763bc7a490e53bd31d5a713158577fa74cfd7b

Author: romannurik

docs/html/distribute/distribute_toc.cs

@@ -74,6 +74,8 @@
         <span class="en">Google Play Badges</a></li>
        <li><a href="<?cs var:toroot ?>distribute/googleplay/promote/brand.html">
         <span class="en">Brand Assets and Guidelines</a></li>
+       <li><a href="<?cs var:toroot ?>distribute/promote/device-art.html">
+        <span class="en">Device Art Generator</a></li>
      </ul>
    </li>