[3.13] Add 'Show translation source' to docs sidebar (GH-130355) (#142687)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Co-authored-by: Maciej Olko <maciej.olko@affirm.com>

Author: 4 people

Doc/tools/templates/customsourcelink.html

@@ -8,6 +8,12 @@ <h3>{{ _('This page') }}</h3>
             rel="nofollow">{{ _('Show source') }}
         </a>
       </li>
+      {% if language != "en" %}
+      <li>
+        <a href="https://github.com/python/python-docs-{{ language }}/blob/{{ version }}/{{ pagename }}.po?plain=1"
+           rel="nofollow">{{ _('Show translation source') }}</a>
+      </li>
+      {% endif %}
     </ul>
   </div>
 {%- endif %}