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">
  {{- 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 -}}
    {{- $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 }}"{{ if $active_group }} aria-current="true"{{ end }}>
        {{ $group.title }}
      </a>

      {{- if $group.pages }}
      <ul class="nav bd-sidenav flex-column 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 }}/"{{ if and $active_group (eq $page_slug $doc_slug) }} aria-current="page"{{ end }}>
              {{- $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/">
        Migration
      </a>
    </li>
  </ul>
</nav>