Make all headers target URL fragment slug if no href is specified.

docs/src/_includes/nodes/Heading.njk

@@ -23,7 +23,7 @@
     {% if node.href %}
       <a href="{{ node.href }}" class="border-b-2 {{ levelLinkClasses[node.level] }}">{{ node.text | safe }}</a>
     {% else %}
-      {{ node.text | safe }}
+      <a href="#{{ node.slug }}">{{ node.text | safe }}</a>
     {% endif %}
   </h{{ node.level }}>
 {% endmacro %}