Add favicon

Author: DecimalTurn

_layouts/default.html

@@ -9,6 +9,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <meta name="theme-color" content="#157878">
     <meta name="apple-mobile-web-app-status-bar-style" content="black-translucent">
+    <link rel="shortcut icon" type="image/png" href="{{ '/favicon.png' | relative_url }}">
     <link rel="stylesheet" href="{{ '/assets/css/style.css?v=' | append: site.github.build_revision | relative_url }}">
     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.11.1/styles/vs2015.min.css">
     <script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.11.1/highlight.min.js"></script>