Add condition for nav exclude

_includes/nav.html

 <nav>
   <ul class="navigation-list">
   {% assign pages_list = site.html_pages | sort:"nav_order" %}
-  {% for node in pages_list %}
+    {% for node in pages_list %}
+    {% unless node.nav_exclude %}
       <li class="navigation-list-item{% if page.url == node.url %} active{% endif %} js-side-nav-item">
         {% if node.parent == nil or node.has_children == true %}
         <a href="{{ node.url | absolute_url }}" class="navigation-list-link{% if page.url == node.url or (page.parent != nil and page.parent == node.parent) %} active{% endif %}">{{ node.title }}</a>
@@ -19,6 +20,7 @@
           {% endif %}
         {% endif %}
       </li>
-  {% endfor %}
+    {% endunless %}
+    {% endfor %}
   </ul>
 </nav>