Add archive docs

Fix #307

docs/content/documentation/templates/archive.md

@@ -0,0 +1,23 @@
++++
+title = "Archive"
+weight = 90
++++
+
+Gutenberg doesn't have a built-in way to display an archive page, a page showing
+all post titles ordered by year. However, this can be accomplished directly in the templates:
+
+```jinja2
+{% for year, posts in section.pages | group_by(attribute="year") %}
+    <h2>{{ year }}</h2>
+
+    <ul>
+    {% for post in posts %}
+        <li><a href="{{ post.permalink }}">{{ post.title }}</a></li>
+    {% endfor %}
+    </ul>
+{% endfor %}
+```
+
+This snippet assumes that posts are sorted by date and that you want to display the archive
+in a descending order. If you want to show articles in a ascending order, simply add a `reverse` filter
+after the `group_by`.