build(aio): improve API docs layout and styling

aio/tools/transforms/templates/api/var.template.html

@@ -1,8 +1,11 @@
 {% extends 'export-base.template.html' %}
 
-{% block details %}
+{% block overview %}
   <code-example language="ts" hideCopy="true" class="no-box api-heading">
   const {$ doc.name $}: {$ doc.symbolTypeName or 'any' $};
   </code-example>
+{% endblock %}
+
+{% block details %}
   {% include "includes/description.html" %}
 {% endblock %}