refactor templates layout

@see https://gohugo.io/templates/new-templatesystem-overview/

layouts/partials/single/main-body/corrigendum.html

@@ -1,37 +0,0 @@
-{{- /*
-  Displays corrigenda for an item
-
-  @params pc  Page context
-  @params sc  Site context
-  */
--}}
-
-
-<!-- Validation -->
-
-{{- if ( not ( isset . "sc" ) ) -}}
-  {{- errorf "No site context received" -}}
-{{- end -}}
-
-{{- if ( not ( isset . "pc" ) ) -}}
-  {{- errorf "No page context received (%q)" .sc.Page.File.Dir -}}
-{{- end -}}
-
-
-<!-- Rendering -->
-
-<section
-  class="article-body__corrigendum"
-  itemprop="correction"
-  itemscope
-  itemtype="https://schema.org/CorrectionComment"
->
-  <h2
-    class="subheading subheading--corrigendum"
-    id="corrigendum"
-  >
-    Corrigendum
-  </h2>
-  {{ $file := path.Join $.sc.Page.File.Dir "corrigendum.md" | readFile }}
-  {{ $file | .pc.RenderString }}
-</section>