Add pre-browsing hints for external services

Author: Munter

docs/_includes/head.html

@@ -2,6 +2,8 @@
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width,initial-scale=1">
   <title>{{ page.title }}</title>
+  <link rel="preconnect" href="https://opencollective.com">
+  <link rel="prefetch" href="https://www.google-analytics.com/analytics.js">
   <link rel="stylesheet" href="css/normalize.css"/>
   <link rel="stylesheet" href="css/style.css"/>
   <link rel="stylesheet" href="css/pygments.css"/>