feat: add auto default section menu

[menu]

  [menu.section]

    auto = true

    collapsibleSidebar = true

    mainSections = ["docs", "tutorial"]

    collapsibleSidebar = false

    mainSections = ["docs"]

<!-- Auto default section menu, single level -->

{{ $currentPage := . -}}

{{ range (where .Site.Sections "Section" "in" site.Params.menu.section.mainSections) }}

  {{ range .Sections.Reverse }}

    {{ $active := in $currentPage.RelPermalink .RelPermalink }}

    <h3 class="h6 text-uppercase mb-2">{{ .Title }}</h3>

    <ul class="list-unstyled">

      {{ range .Pages }}

        {{ if .IsNode }}

          {{ $active := in $currentPage.RelPermalink .RelPermalink }}

          <h4 class="h6 text-uppercase ms-3 mt-3 mb-2">{{ .Title }}</h4>

          <ul class="list-unstyled ms-3">

            {{ range .Pages }}

              {{ if .IsNode }}

                {{ $active := in $currentPage.RelPermalink .RelPermalink }}

                <h5 class="h6 text-uppercase mt-2 mb-2">{{ .Title }}</h5>

                <ul class="list-unstyled ms-3">

                  {{ range .Pages }}

                    {{ $active := in $currentPage.RelPermalink .RelPermalink }}

                    <li><a class="docs-link{{ if $active }} active{{ end }}" href="{{ .URL | relURL }}">{{ .Name }}</a></li>

                  {{ end }}

                </ul>

              {{ else }}

                {{ $active := in $currentPage.RelPermalink .RelPermalink }}

                <li><a class="docs-link{{ if $active }} active{{ end }}" href="{{ .URL | relURL }}">{{ .Name }}</a></li>

              {{ end }}

            {{ end }}

          </ul>

        {{ else }}

          {{ $active := in $currentPage.RelPermalink .RelPermalink }}

          <li><a class="docs-link{{ if $active }} active{{ end }}" href="{{ .URL | relURL }}">{{ .Name }}</a></li>

        {{ end }}

      {{ end }}

    </ul>

  {{ end }}

{{ end }}

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