Bootstrap/_includes/docs-sidebar.html

<form class="bd-search d-flex align-items-center">
  <input type="search" class="form-control" id="search-input" placeholder="Search..." aria-label="Search for..." autocomplete="off">
  <button class="btn-link bd-search-docs-toggle d-md-none p-0 ml-3" type="button" data-toggle="collapse" data-target="#bd-docs-nav" aria-controls="bd-docs-nav" aria-expanded="false" aria-label="Toggle docs avigation">
    {% include icons/menu.svg class="" width="30" height="30" %}
  </button>
</form>

<nav class="collapse bd-links" id="bd-docs-nav">
  {% assign page_slug = page.url | split: '/' | last %}
  {% for group in site.data.nav %}
  {% assign link = group.pages | first %}
  {% assign link_slug = link.title | slugify %}
  {% assign group_slug = group.title | slugify %}
  {% assign active = nil %}

  {% if page.group == group_slug %}
    {% assign active = 'active' %}
  {% endif %}

  <div class="bd-toc-item {{ active }}">
      <a class="bd-toc-link" href="{{ site.baseurl }}/docs/{{ site.docs_version }}/{{ group_slug }}/{{ link_slug }}{% if link_slug %}/{% endif %}">
        {{ group.title }}
      </a>

      <ul class="nav bd-sidenav">
        {% for doc in group.pages %}
          {% assign doc_slug = doc.title | slugify %}
          {% assign active = nil %}

          {% if page.group == group_slug and page_slug == doc_slug %}
            {% assign active = 'active bd-sidenav-active' %}
          {% endif %}

          <li class="{{ active }}">
            <a href="{{ site.baseurl }}/docs/{{ site.docs_version }}/{{ group_slug }}/{{ doc_slug }}/">
              {{ doc.title }}
            </a>

            {% comment %}
            {% unless doc.sections == nil %}
              <ul class="nav">
                {% for section in doc.sections %}
                  <li>
                    <a href="#{{ section.title | downcase | replace: ' ', '-' }}">
                      {{ section.title }}
                    </a>
                  </li>
                {% endfor %}
              </ul>
            {% endunless %}
            {% endcomment %}
          </li>
        {% endfor %}
      </ul>
    </div>
  {% endfor %}
</nav>