[fix] sphinx 5.x: add nav.contents everywhere that div.topic is used

Previously, docutils produced `div.topic` for the contents directive, the latest
version produces `nav.contents`.  This means that those tables of contents
change appearance when switching to docutils 0.18 [1][2].

[1] sphinx-doc/sphinx@5806f0a
[2] sphinx-doc/sphinx#10534

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/_themes/searxng/static/searxng.css

@@ -40,7 +40,7 @@ p.sidebar-title, .sidebar p {
 /* admonitions
 */
 
-div.admonition, div.topic, div.toctree-wrapper {
+div.admonition, div.topic, nav.contents, div.toctree-wrapper {
   background-color: #fafafa;
   margin: 8px 0px;
   padding: 1em;