Add Matomo Tracking to Docs site.

Matomo is an Open Source web analytics platform that allows us
to get better insights and optimize for our audience without
the negative consequences of other compareable platforms.

Read more: https://matomo.org/why-matomo/

docs/_static/js/matomo.js

@@ -0,0 +1,11 @@
+var _paq = window._paq || [];
+/* tracker methods like "setCustomDimension" should be called before "trackPageView" */
+_paq.push(['trackPageView']);
+_paq.push(['enableLinkTracking']);
+(function() {
+  var u="https://matomo.ethereum.org/piwik/";
+  _paq.push(['setTrackerUrl', u+'matomo.php']);
+  _paq.push(['setSiteId', '20']);
+  var d=document, g=d.createElement('script'), s=d.getElementsByTagName('script')[0];
+  g.type='text/javascript'; g.async=true; g.defer=true; g.src=u+'matomo.js'; s.parentNode.insertBefore(g,s);
+})();

docs/conf.py

@@ -89,6 +89,9 @@
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = ['_static']
 
+def setup(app):
+    app.add_js_file("js/matomo.js")
+
 # Custom sidebar templates, must be a dictionary that maps document names
 # to template names.
 #