refactor templates layout

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

layouts/_partials/single/main-body/appendices.html

@@ -0,0 +1,32 @@
+{{- /*
+  Displays appendices 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__appendices">
+  <h2
+    class="subheading subheading--appendices"
+    id="appendices"
+  >
+    Appendices
+  </h2>
+  {{ $file := path.Join $.sc.Page.File.Dir "appendices.md" | readFile }}
+  {{ $file | .pc.RenderString }}
+</section>