Add GitHub Pages favicon

Author: masarray

docs/index.html

@@ -10,6 +10,8 @@
   <meta property="og:description" content="A clean open-source learning gateway for routing IEC 61850 values into Modbus TCP and MQTT for HMI, DCS, and IoT labs." />
   <meta property="og:type" content="website" />
   <meta property="og:image" content="assets/gateway-hero.webp" />
+  <link rel="icon" type="image/png" sizes="256x256" href="assets/arserver-icon.png" />
+  <link rel="apple-touch-icon" href="assets/arserver-icon.png" />
   <link rel="preload" as="image" href="assets/gateway-hero.webp" />
   <link rel="stylesheet" href="styles.css" />
 </head>