website: Link to the v0.15 upgrade guide

Unfortunately it seems that this link got lost in a merge conflict when
we did the big nav refactor earlier in the v0.15 cycle, so here we'll
retoractively add it to the new location for upgrade guide nav, in the
language layout rather than the downloads layout.

website/layouts/language.erb

@@ -971,6 +971,10 @@
             <a href="/upgrade-guides/index.html">Overview</a>
           </li>
 
+          <li>
+            <a href="/upgrade-guides/0-15.html">Upgrading to v0.15</a>
+          </li>
+
           <li>
             <a href="/upgrade-guides/0-14.html">Upgrading to v0.14</a>
           </li>