UX: make theme compatible with AI gists (#140)

Before (only pinned topic excerpts, no gist toggle):

![image](https://github.com/user-attachments/assets/7a1b78b8-e115-4c96-9ebf-52d2c1237eea)


After (gists can be toggled on): 

![image](https://github.com/user-attachments/assets/9e6786be-3817-4420-8142-1c5f7cad54ea)


This puts the gists in the same place as the theme places excerpts and
uses the same styles. It also follows the theme pattern of hiding
excerpts/gists on mobile.

scss/misc.scss

@@ -69,8 +69,11 @@ input[type="color"]:focus,
   background-color: var(--background-color);
 }
 
-.fk-d-menu__trigger.topic-list-layout-trigger {
-  display: none;
+@include breakpoint(mobile-extra-large) {
+  // pinned topic excerpts are hidden on small screens too
+  .fk-d-menu__trigger.topic-list-layout-trigger {
+    display: none;
+  }
 }
 
 .search-container .search-header,