[Website]: Let menu to be above add (vuejs#1188) (vuejs#384)

Signed-off-by: Bruno Lesieur <bruno.lesieur@gmail.com>

Author: vuejs-jp-bot

themes/vue/source/css/page.styl

@@ -192,7 +192,7 @@
   .content.with-sidebar
     margin-left: 290px
   #ad
-    z-index: 7
+    z-index: 1
     position: relative
     padding: 0
     bottom: 0