feat(admonitions): add header-only support

assets/css/compiled/main.css

@@ -1452,6 +1452,11 @@
   .self-center {
     align-self: center;
   }
+  .truncate {
+    overflow: hidden;
+    text-overflow: ellipsis;
+    white-space: nowrap;
+  }
   .\!overflow-hidden {
     overflow: hidden !important;
   }