scss: Add support for h6 counting

Author: gonX

site/assets/css/site.scss

@@ -142,7 +142,7 @@ div.highlighter-rouge {
     @include markdown-header;
   }
 
-  h2, h3, h4, h5 {
+  h2, h3, h4, h5, h6 {
     // counter display
     > span::before {
       font-style: italic;