use the unstyled menu list on mobile

The dropdown looks great on full size, but when we switch to the mobile
view, the unstyled list actually looks better. This breakpoint matches
the `navbar-expand-lg` parent class, so if that's changed, then this
should change to match. See https://getbootstrap.com/docs/5.0/layout/breakpoints/#available-breakpoints

static/css/site.css

@@ -51,37 +51,42 @@ ul.docs-index li:hover {
   padding-top: 30px;
 }
 
-.navbar .submenu {
-  position: relative;
-  display: inline-block;
-  min-width: 8em;
-}
-  .navbar .submenu .nav-header {
+/* The unstyled list actually looks better on mobile. This width matches the
+   `navbar-expand-lg` parent class. If you change that, then change this to match.
+    See https://getbootstrap.com/docs/5.0/layout/breakpoints/#available-breakpoints
+ */
+@media (min-width: 991.98px) {
+  .navbar .submenu {
+    position: relative;
     display: inline-block;
-    padding: 0.5em;
-  }
-  .navbar .submenu .submenu-content {
-    display: none;
-    position: absolute;
-    z-index: 100;
-    background-color: #3d8bfd;
-    min-width: 15em;
-    padding-left: 0.25em;
-    box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
-  }
-  .navbar .submenu .submenu-content li {
-    list-style-type: none;
-  }
-  .navbar .submenu .submenu-content li i {
-    min-width: 1.25em;
+    min-width: 8em;
   }
-  .navbar .submenu:hover .submenu-content {
-    display: block;
-  }
-  .navbar .submenu:hover .nav-header {
-    background-color: #3d8bfd;
-  }
-
+    .navbar .submenu .nav-header {
+      display: inline-block;
+      padding: 0.5em;
+    }
+    .navbar .submenu .submenu-content {
+      display: none;
+      position: absolute;
+      z-index: 100;
+      background-color: #3d8bfd;
+      min-width: 15em;
+      padding-left: 0.25em;
+      box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
+    }
+    .navbar .submenu .submenu-content li {
+      list-style-type: none;
+    }
+    .navbar .submenu .submenu-content li i {
+      min-width: 1.25em;
+    }
+    .navbar .submenu:hover .submenu-content {
+      display: block;
+    }
+    .navbar .submenu:hover .nav-header {
+      background-color: #3d8bfd;
+    }
+}
 
 div.mermaid {
   border: 1px solid #aaa;