sidebar: make TOC collapsible with <details>

Use semantic HTML to make the table of contents in the sidebar
collapsible.

Author: To1ne

layouts/partials/sidebar.html

@@ -53,15 +53,15 @@
 
   {{ $headings := .Params.headings }}
   {{ if $headings }}
-  <ul id="toc">
-    <li> Table of Contents
-      <ul class="expanded">
+  <details id="toc" open>
+    <summary>Table of Contents</summary>
+    <ul>
         {{ range $i, $item := $headings }}
           <li><a href="#{{ .id }}">{{ .text }}</a> </li>
         {{ end }}
       </ul>
     </li>
-  </ul>
+  </details>
   {{ end }}
 
   {{ if (eq .Params.Sidebar "book") }}