Add table of contents link to orphan doc: `other/environmental-variables.html`.

website/source/layouts/docs.erb

@@ -69,6 +69,7 @@
 			<li><h4>Other</h4></li>
 			<li><a href="/docs/other/core-configuration.html">Core Configuration</a></li>
 			<li><a href="/docs/other/debugging.html">Debugging</a></li>
+			<li><a href="/docs/other/environmental-variables.html">Environmental Variables for Packer</a></li>
 		</ul>
 
 		<ul>