Add 2nd level namespaces to the left nav

subdoc/gen_tests/nested-namespace/index.html

@@ -263,6 +263,9 @@
           <li>
             <a class="sidebar-item" href="namespace.outer_namespace.html">outer_namespace</a>
           </li>
+          <li>
+            <a class="sidebar-item nested" href="outer_namespace-namespace.inner_namespace.html">inner_namespace</a>
+          </li>
         </ul>
       </div>
     </nav>

subdoc/gen_tests/subdoc-test-style.css

@@ -166,6 +166,9 @@ nav.sidebar .sidebar-links li > a.sidebar-item {
     color: rgb(250, 199, 121);
     font-size: 1.0em;
 }
+nav.sidebar .sidebar-item.nested {
+    margin-left: 0.75rem;
+}
 @media (max-width: 700px) {
     nav.sidebar {
         display: none;

subdoc/gen_tests/typenames-across-paths/index.html

@@ -266,6 +266,9 @@
           <li>
             <a class="sidebar-item" href="namespace.other.html">other</a>
           </li>
+          <li>
+            <a class="sidebar-item nested" href="other-namespace.subother.html">subother</a>
+          </li>
         </ul>
       </div>
     </nav>

subdoc/lib/gen/generate_namespace.cc

@@ -766,6 +766,23 @@ sus::Result<void, MarkdownToHtmlError> generate_namespace(
           namespace_element_from_sorted(element, sorted_ns);
       sidebar_links.push(SidebarLink(SidebarLinkStyle::Item, ne.name,
                                      construct_html_url_for_namespace(ne)));
+
+      Vec<SortedNamespaceByName> sorted;
+      for (const auto& [key, sub_element] : ne.namespaces) {
+        if (sub_element.hidden()) continue;
+        if (sub_element.is_empty()) continue;
+
+        sorted.push(sus::tuple(sub_element.name, sub_element.sort_key, key));
+      }
+      sorted.sort_unstable_by(cmp_namespaces_by_name);
+
+      for (const SortedNamespaceByName& sub_sorted_ns : sorted) {
+        const NamespaceElement& sub_ne =
+            namespace_element_from_sorted(ne, sub_sorted_ns);
+        sidebar_links.push(
+            SidebarLink(SidebarLinkStyle::NestedItem, sub_ne.name,
+                        construct_html_url_for_namespace(sub_ne)));
+      }
     }
   }
   if (!sorted_classes.is_empty()) {

subdoc/lib/gen/generate_nav.cc

@@ -114,6 +114,10 @@ void generate_nav(HtmlWriter::OpenBody& body, const Database& db,
             a.add_class("sidebar-header");
             break;
           case SidebarLinkStyle::Item: a.add_class("sidebar-item"); break;
+          case SidebarLinkStyle::NestedItem:
+            a.add_class("sidebar-item");
+            a.add_class("nested");
+            break;
         }
         a.add_href(link.href);
         a.write_text(link.text);

subdoc/lib/gen/generate_nav.h

@@ -26,6 +26,7 @@ enum class SidebarLinkStyle {
   Parent,
   GroupHeader,
   Item,
+  NestedItem,
 };
 
 struct SidebarLink {