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/_how-to-use.html

@@ -0,0 +1,9 @@
+{%- if doc.howToUse %}
+  <!-- QUICK REFERENCE CODE EXAMPLE -->
+  <section class="how-to-use">
+    <h2>How To Use</h2>
+    <div class="description">
+      {$ doc.howToUse | marked $}
+    </div>
+  </section>
+{% endif %}