[docs] Use subitem in new nav generation

(cherry picked from commit 995d0e5)

Author: zpao

docs/_includes/nav_docs.html

@@ -11,7 +11,7 @@ <h3>{{ section.title }}</h3>
               <ul>
                 {% for subitem in item.subitems %}
                   <li>
-                    {{ item | sidebar_item_link}}
+                    {{ subitem | sidebar_item_link}}
                   </li>
                 {% endfor %}
               </ul>