refactor: split single header and meta components into partials

layouts/partials/single/main-header/toc.html

@@ -0,0 +1,34 @@
+{{- /*
+  Renders appendices and corrigenda for an item.
+
+  @params toc  The page ToC.
+  @params appendices    The item's appendices. Optional.
+  @params fileDir   The file folder name.
+  */
+-}}
+
+{{ $empty_toc_html := "<nav id=\"TableOfContents\"></nav>" }}
+
+{{ if and ( .toc ) ( ne .toc $empty_toc_html ) }}
+  <nav class="article-header__table-of-contents">
+    <h3 class="article-header__subtitle">Table of Contents</h3>
+    {{ .toc }}
+    {{- if fileExists ( path.Join .fileDir "appendices.md" ) -}}
+      <ul class="toc-list--appendices">
+        <li>
+          <a href="#appendices">Appendices</a>
+          <ol>
+            {{ range .appendices }}
+              <li><a href="#{{ . | anchorize }}">{{ . | safeHTML }}</a></li>
+            {{ end }}
+          </ol>
+        </li>
+      </ul>
+    {{- end -}}
+    {{- if fileExists ( path.Join .fileDir "corrigenda.md" ) -}}
+      <ul class="toc-list--corrigenda">
+        <li><a href="#corrigenda">Corrigenda</a></li>
+      </ul>
+    {{- end -}}
+  </nav>
+{{ end }}