Add preconnect for doubleclick domain that google analytics results in contacting

Author: Munter

docs/_includes/head.html

@@ -4,6 +4,7 @@
   <title>{{ page.title }}</title>
   <link rel="preconnect" href="https://opencollective.com">
   <link rel="preconnect" href="https://www.google-analytics.com">
+  <link rel="preconnect" href="https://stats.g.doubleclick.net">
   <link rel="stylesheet" href="css/normalize.css"/>
   <link rel="stylesheet" href="css/style.css"/>
   <link rel="stylesheet" href="css/pygments.css"/>