Add short-code to list (nested) pages of the current section

Adds a new short-code to display all (nested) section pages in a link list (with
header title) with nested link lists for nested page sections. The order of the
list items resembles the page order in the navigation menu.

Usage:

Use the short-code in the markdown page the represents a subsection (a
subsection is represented by a markdown page and a subsection directory
(identically named) that resides in the same directory as the markdown page).

```
{{< section-toc title="Contents">}}
```

Note that the named input argument `title` is optional and defaults to
`Contents`.

Signed-off-by: Sietze van Buuren <Sietze.vanBuuren@de.bosch.com>

themes/apertis/layouts/partials/subsection-listing.html

+{{- $order := cond $.sort_by_weight "weight" "sort_title" -}}
+{{- range sort $.files $order -}}
+    {{- $show_page := (cond $.sort_by_weight (cond $.weighted_top (and (lt .weight 100) (ne .weight nil) ) (gt .weight 100) ) (or (eq .weight 100) (eq .weight nil) ) ) -}}
+    {{- if $show_page -}}
+        {{- if not (in (slice "index" "_index") .file) -}}
+            <li><a href="{{ .link }}">{{ .title }}</a></li>
+        {{- end -}}
+        {{- range (where $.dirs "Name" "eq" .file) }}
+            {{- partial "subsection-pages.html" (dict "context" $.context "site" $.site "link" (print $.path .Name "/")) -}}
+        {{- end -}}
+    {{- end -}}
+{{- end -}}

themes/apertis/layouts/partials/subsection-pages.html

+{{- $path := $.link -}}
+{{- $ls := readDir (print "content/" $path) -}}
+{{- $dirs := where $ls "IsDir" true -}}
+{{- $files := slice -}}
+{{- range (where $ls "IsDir" false) }}
+    {{- $file := strings.TrimSuffix ".md" .Name -}}  
+    {{- $fpath := print $path .Name -}}
+    {{- $link := strings.TrimSuffix ".md" $fpath -}}
+    {{- with $.context.Site.GetPage "filepath" $fpath -}}
+        {{$files = $files | append (dict "name" .Name "title" .Title "sort_title" (print "sort_" .Title) "weight" (cond (eq .Weight 0) nil .Weight) "file" $file "fpath" $fpath "link" $link) }}
+    {{- else -}}
+    {{- end -}}
+{{- end -}}
+<ul>
+{{- partial "subsection-listing.html" (dict "context" $.context "files" $files "path" $path "dirs" $dirs "sort_by_weight" true "weighted_top" true) -}}
+{{- partial "subsection-listing.html" (dict "context" $.context "files" $files "path" $path "dirs" $dirs "sort_by_weight" false "weighted_top" false) -}}
+{{- partial "subsection-listing.html" (dict "context" $.context "files" $files "path" $path "dirs" $dirs "sort_by_weight" true "weighted_top" false) -}}
+</ul>
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themes/apertis/layouts/shortcodes/section-toc.html

+{{- $title := .Get "title" | default "Contents" -}}
+<h3>{{ $title }}</h3>
+{{- partial "subsection-pages.html" (dict "context" . "link" .Page.RelPermalink) -}}
\ No newline at end of file