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/_export-as.html

@@ -0,0 +1,10 @@
+{%- if doc.directiveOptions.exportAs %}
+<section class="export-as">
+  <h2>Exported as</h2>
+  <div class="description">
+    <p>
+      <code>{$ doc.directiveOptions.exportAs $}</code>
+    </p>
+  </div>
+</section>
+{% endif %}