Add a link to our backwards compatibility policy to our side-bar

It is important enough that it should be easier to find

doc/en/_templates/globaltoc.html

@@ -9,6 +9,7 @@
   <li><a href="{{ pathto('contact') }}">Contact</a></li>
   <li><a href="{{ pathto('talks') }}">Talks/Posts</a></li>
   <li><a href="{{ pathto('changelog') }}">Changelog</a></li>
+  <li><a href="{{ pathto('backwards-compatibility') }}">Backwards Compatibility</a></li>
   <li><a href="{{ pathto('license') }}">License</a></li>
 </ul>