add GoatCounter analytics and Privacy Policy

- fix #42
- see MDAnalysis/MDAnalysis.github.io#129 for Privacy Policy

Author: orbeckst

docs/_templates/footer.html

@@ -0,0 +1,11 @@
+{% extends "!footer.html" %}
+
+{% block extrafooter %}
+{{ super() }}
+<div class="footer"><p>Please see
+    our <a href="https://www.mdanalysis.org/pages/privacy/">Privacy Policy</a>
+    to learn how <a href="https://www.mdanalysis.org">MDAnalysis</a> collects data.</p>
+    <script data-goatcounter="https://mdanalysis.goatcounter.com/count"
+        async src="//gc.zgo.at/count.js"></script>
+</div>
+{% endblock %}