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

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

  <ul class="list-unstyled mb-0 pt-1 pb-3">
  {{- range $group := .Site.Data.sidebar -}}
    {{- $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 -}}
    {{- $is_active_group := eq $.Page.Params.group $group_slug }}

    <li class="mb-1{{ if $is_active_group }} active{{ end }}">
      <a class="d-inline-flex align-items-center rounded{{ if not $is_active_group }} collapsed{{ end }}" data-toggle="collapse" href="#{{ $group_slug }}-collapse" role="button" aria-expanded="{{ $is_active_group }}"{{ if $is_active_group }} aria-current="true"{{ end }}>
        {{ $group.title }}
      </a>

      {{- if $group.pages }}
      <div class="collapse{{ if $is_active_group }} show{{ end }}" id="{{ $group_slug }}-collapse">
        <ul class="list-unstyled font-weight-normal pb-1 small">
          {{- range $doc := $group.pages -}}
            {{- $doc_slug := $doc.title | urlize -}}
            {{- $is_active := and $is_active_group (eq $page_slug $doc_slug) -}}
            {{- $href := printf "/docs/%s/%s/%s/" $.Site.Params.docs_version $group_slug $doc_slug }}
            <li><a href="{{ $href }}" class="d-inline-flex align-items-center rounded{{ if $is_active }} active{{ end }}"{{ if $is_active }} aria-current="page"{{ end }}>{{ $doc.title }}</a></li>
          {{- end }}
        </ul>
      </div>
      {{- end }}
    </li>
  {{- end }}

    <li class="my-3 mx-4 border-top"></li>
    <li{{ if eq $page_slug "migration" }} class="active"{{ end }}>
      <a href="/docs/{{ $.Site.Params.docs_version }}/migration/" class="d-inline-flex align-items-center rounded">
        Migration
      </a>
    </li>
  </ul>
</nav>