Bootstrap/site/_includes/docs-sidebar.html

<form role="search" 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" data-docs-version="{{ site.docs_version }}">
  <button class="btn 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 navigation">
    {%- include icons/menu.svg width="30" height="30" -%}
  </button>
</form>

<nav class="collapse bd-links" id="bd-docs-nav" aria-label="Main navigation">
  {%- 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{% unless active == nil %} {{ active }}{% endunless %}">
      <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{% unless active == nil %} class="{{ active }}"{% endunless %}>
            <a href="{{ site.baseurl }}/docs/{{ site.docs_version }}/{{ group_slug }}/{{ doc_slug }}/">
              {{ doc.title }}
            </a>
          </li>
        {%- endfor -%}
      </ul>
    </div>
  {%- endfor -%}
</nav>