feat: allow customizable heading levels for details summary

src/_includes/partials/details.njk

@@ -1,3 +1,5 @@
+{% set headingLevel = headingLevel | default(false) %}
+
 <div class="details flow">
   <div class="control | cluster" aria-label="{{ meta.details.aria }}">
     <button id="expandAll" class="button" data-small-button>{{ meta.details.expand }}</button>
@@ -5,7 +7,13 @@
   </div>
   {%- for item in itemList | alphabetic -%}
     <details id="{{ item.data.title | slugify }}">
-      <summary>{{ item.data.title }}</summary>
+      <summary>
+        {%- if headingLevel -%}
+          <{{ headingLevel }}>{{ item.data.title }}</{{ headingLevel }}>
+        {%- else -%}
+          {{ item.data.title }}
+        {%- endif -%}
+      </summary>
       {{- item.templateContent | safe -}}
     </details>
   {%- endfor -%}