update index.html template to match page and archive entries styling.

The index.html is the default home page and had it's own styling. Based on the
feedback provided by Nic, regarding the layout in the page entry HTML templates,
I've added the HTML tags and CSS classes to the index.html template. This is me
just getting ahead of a change she is going to ask for in the future.

It is a minor change so changing it back won't be a problem.

templates/index.html

@@ -1,7 +1,9 @@
 {% extends "layouts/default.html" %}
 {% block title %}{{system-data.first.site-name}}{% endblock %}
 {% block content %}
-<main>
-  {{content | safe }}
-</main>
+<article class="fe-article">
+    <div class="fe-article-body">
+        {{content | safe }}
+    </div>
+</article>
 {% endblock %}