Insert Plausible.io analytics code into docs

docs/_templates/layout.html

@@ -0,0 +1,6 @@
+{% extends "!layout.html" %}
+
+{% block footer %}
+{{ super() }}
+<script async defer data-domain="docs.wagtail.io" src="https://plausible.io/js/plausible.js"></script>
+{% endblock %}