Fix CSS stylesheet link for GitHub Pages

Author: emilbaehr

_includes/head.html

@@ -16,4 +16,6 @@
 
 	<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.6.3/css/all.css" integrity="sha384-UHRtZLI+pbxtHCWp1t77Bi1L4ZtiqrqD80Kn4Z8NTSRyMA2Fd33n5dQ8lWUE00s/" crossorigin="anonymous">
 	<link rel="stylesheet" href="/main.css">
+	<link rel="stylesheet" href="main.css">
+	<link rel="stylesheet" href="../main.css">
 </head>