build(aio): remove formatting and styles from dgeni templates

Much of the formatting was hardcoded and copied from the old anguar.io
jade files. This gives us a clean start.

Also, more use has been made of include files to make the templates
easier to understand and manage.

docs/templates/includes/_description.html

@@ -0,0 +1,9 @@
+<section class="l-content-block">
+  <h3 class="text-display-1" id="api-description">
+    Description
+    <a href="#api-description" class="header-link"><i class="material-icons">link</i></a>
+  </h3>
+  {%- if doc.description.length > 2 %}
+  {$ doc.description | trimBlankLines | marked $}
+  {% endif %}
+</section>