notes/_includes/nav.html

<nav>
  <ul class="navigation-list">
  {% assign pages_list = site.html_pages | sort:"nav_order" %}
  {% for node in pages_list %}
      <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>
          {% if (node.has_children == true and node.parent == page.parent) %}
            {% assign children_list = site.html_pages | sort:"nav_order" %}
            <ul class="navigation-list-child-list">
              {% for child in children_list %}
                {% if child.parent == node.parent and child.title != node.title %}
                <li class="navigation-list-item {% if page.url == child.url %} active{% endif %}">
                  <a href="{{ child.url | absolute_url }}" class="navigation-list-link{% if page.url == child.url %} active{% endif %}">{{ child.title }}</a>
                </li>
                {% endif %}
              {% endfor %}
            </ul>
          {% endif %}
        {% endif %}
      </li>
  {% endfor %}
  </ul>
</nav>