change content behaviour, change open-element to collapsible, several fixes

layouts/shortcodes/collapsible.html

@@ -0,0 +1,10 @@
+<div class="container mb-0">
+  <div class="card border-primary rounded-0 hover-shadow mb-5">
+    <div class="card-body mb-0">
+      <details class="mb-0">
+      <summary class="mb-0">{{ .Get "title" }}</summary>
+      <div class="content"><p class="content">{{ .Inner }}</p></div>
+      </details>
+    </div>
+  </div>
+</div>