add number for mobile toc, fix order for nested titles (#555)

Co-authored-by: Alex Shpak <[email protected]>

assets/plugins/_numbered.scss

@@ -20,13 +20,11 @@ $endLevel: 6;
 }
 
 .book-toc nav#TableOfContents ul {
+  counter-reset: item;
+
   li {
     counter-increment: item;
 
-    &:first-child {
-      counter-reset: item;
-    }
-
     &:before {
       content: counters(item, ".") ". ";
       float: left;