Mirror/docs/_includes/sidebar.html

<div  class="col-12 col-md-3 col-xl-2 bd-sidebar">
    
    <nav class="collapse bd-links" id = "sidebar">

        {% for category in site.data.navigation %}
            {% assign catid = forloop.index %}
            <div class="bd-toc-item">
                <a href="#submenu_{{catid}}" aria-controls="submenu_{{catid}}" data-toggle="collapse" aria-expanded="false" class="bd-toc-link dropdown-toggle">{{category.title}}</a>
            
                <ul class="collapse nav bd-sidenav" id="submenu_{{catid}}">
                    
                    {% for document in category.children %}  
                        {% assign docid = forloop.index %}                 
                        <li>
                            
                            {% if document.children %}
                                <a href="#submenu_{{catid}}_{{docid}}" aria-controls="submenu_{{catid}}_{{docid}}" data-toggle="collapse" aria-expanded="false" class="bd-toc-link dropdown-toggle">{{category.title}}</a>
                                <ul class="collapse nav bd-sidenav" id="submenu_{{catid}}_{{docid}}">
                                    {% for child in document.children %}  
                                        <li>
                                            <a href="{{child.url | relative_url}}">{{child.title}}</a>
                                        </li>
                                    {% endfor %}
                                </ul>
                            {% else %}
                                <a href="{{document.url | relative_url}}">{{document.title}}</a>

                            {% endif %}
                        </li>
                    {% endfor %}
                </ul>
            </div>
        {% endfor %}

    </nav>

</div>