build(aio): render titles correctly in content pages

If a document provided a title jsdoc tag then its h1
element was being rendered incorrectly as a markdown
paragraph.

This change renders the title as a markdown h1 tag
directly.

Fixes #16099

aio/tools/transforms/templates/content.template.html

@@ -1,4 +1,4 @@
-{% if doc.title %}<h1>{$ doc.title | marked $}</h1>{% endif %}
+{% if doc.title %}{$ ('# ' + doc.title.trim()) | marked $}{% endif %}
 <div class="content">
 {$ doc.description | marked $}
 </div>