chore(doc-gen): add specific template for displaying variable exports

Closes #1071

docs/dgeni-package/templates/var.template.html

@@ -0,0 +1,8 @@
+{% extends 'layout/base.template.html' %}
+
+{% block body %}
+<h1>{$ doc.name $} <span class="type">variable</span></h1>
+<p class="module">exported from <a href="/{$ doc.moduleDoc.path $}">{$ doc.moduleDoc.id $}</a></p>
+<p>{$ doc.description | marked $}</p>
+
+{% endblock %}