Add Google AdSense to demo site

docs/_layouts/default.html

@@ -15,6 +15,18 @@
 
     {{ content }}
 
+    <script async src="//pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
+    <div align="center" style="margin: 1em 0;">
+    <ins class="adsbygoogle"
+         style="display:block; border-bottom: initial;"
+         data-ad-client="ca-pub-7328585512091257"
+         data-ad-slot="3049671934"
+         data-ad-format="auto"></ins>
+    </div>
+    <script>
+    (adsbygoogle = window.adsbygoogle || []).push({});
+    </script>
+
     <div class="page__footer">
       <footer>
         {% include footer/custom.html %}