website: Add link to heroku_cert in the docs

Author: armon

website/source/layouts/heroku.erb

@@ -21,6 +21,10 @@
 					<a href="/docs/providers/heroku/r/app.html">heroku_app</a>
                     </li>
 
+                    <li<%= sidebar_current("docs-heroku-resource-cert") %>>
+					<a href="/docs/providers/heroku/r/cert.html">heroku_cert</a>
+                    </li>
+
                     <li<%= sidebar_current("docs-heroku-resource-domain") %>>
 					<a href="/docs/providers/heroku/r/domain.html">heroku_domain</a>
                     </li>