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/_selectors.html

@@ -0,0 +1,12 @@
+{%- if doc.directiveOptions.selector.split(',').length %}
+<section class="selectors">
+  <h2>Selectors</h2>
+  <div class="description">
+    {% for selector in doc.directiveOptions.selector.split(',') %}
+    <p class="selector">
+      <code>{$ selector $}</code>
+    </p>
+    {% endfor %}
+  </div>
+</section>
+{% endif %}