Docs: add analytics

docs_overrides/main.html

 <meta property="robots" content="index, follow">
 {% endblock %}
 
+{% block libs %}
+{{ super() }}
+<script data-goatcounter="https://martins-js.goatcounter.com/count" async src="//gc.zgo.at/count.js"></script>
+{% endblock %}
+
 {% block _announce %}
 <a href="#" target="_blank" style="color:white"></a>
 {% endblock %}