Bootstrap/site/layouts/partials/docs-sidebar.html

<nav class="collapse bd-links pb-3" id="bd-docs-nav" aria-label="Main navigation">
  {{- $url := split .Permalink "/" -}}
  {{- $page_slug := index $url (sub (len $url) 2) -}}

  <ul class="list-unstyled">
  {{- range $group := .Site.Data.nav -}}
    {{- $link := $group.title -}}
    {{- $link_slug := $link | urlize -}}

    {{- if $group.pages -}}
      {{- $link = index $group.pages 0 -}}
      {{- $link_slug = $link.title | urlize -}}
    {{- end -}}

    {{- $group_slug := $group.title | urlize -}}
    {{- $active_group := eq $.Page.Params.group $group_slug }}

      <li class="bd-sidenav-group my-1{{ if $active_group }} active{{ end }} js-sidenav-group{{ if $group.pages }} has-children{{ end }}">
        <a class="d-inline-flex align-items-center bd-sidenav-group-link" href="/docs/{{ $.Site.Params.docs_version }}/{{ $group_slug }}/{{ if $group.pages }}{{ $link_slug }}/{{ end }}">
          <div>{{- $group.title -}}</div>
        </a>

        {{- if $group.pages }}
        <ul class="nav bd-sidenav mb-2">
          {{- range $doc := $group.pages -}}
            {{- $doc_slug := $doc.title | urlize }}
            <li{{ if and $active_group (eq $page_slug $doc_slug) }} class="active bd-sidenav-active"{{ end }}>
              <a href="/docs/{{ $.Site.Params.docs_version }}/{{ $group_slug }}/{{ $doc_slug }}/">
                {{- $doc.title -}}
              </a>
            </li>
          {{- end }}
        </ul>
        {{- end }}
      </li>
    {{- end }}

    <li class="my-3 mx-4 border-top"></li>
    <li class="bd-sidenav-group pl-3">
      <a class="d-inline-flex align-items-center bd-sidenav-group-link" href="/docs/{{ $.Site.Params.docs_version }}/migration/">
        <div>Migration</div>
      </a>
    </li>
  </ul>
</nav>