Add canonical link support to 2.4 theme (#53801)

* Add canonical link support to 2.4 theme
* fix so 2.4 docs build

docs/docsite/_themes/srtd/layout.html

@@ -33,6 +33,7 @@
   {% if favicon %}
     <link rel="shortcut icon" href="{{ pathto('_static/' + favicon, 1) }}"/>
   {% endif %}
+  <link rel="canonical" href="https://docs.ansible.com/ansible/latest/{{ pagename }}.html"/>
 
   {# CSS #}
   <link href='https://fonts.googleapis.com/css?family=Lato:400,700|Roboto+Slab:400,700|Inconsolata:400,700' rel='stylesheet' type='text/css'>

docs/templates/plugin.rst.j2

@@ -194,7 +194,7 @@ Common return values are documented here :doc:`common_return_values`, the follow
     <th class="head">type</th>
     <th class="head">sample</th>
     </tr>
-{% for entry in returndocs|dictsort %}
+{% for entry, unusedvalue in returndocs|dictsort %}
 
     <tr>
     <td>@{ entry }@</td>