Add favicon

Same badge as Kestrel's Hall with a B, linked from the index and the
Green Man page.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GrX9qMoNihcWLX3xoJhabw

index.html

@@ -4,6 +4,7 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>Kestrel's Builds</title>
+<link rel="icon" href="favicon.svg">
 <meta name="description" content="Things I've made, written up so you can make them too. Each build comes with a guide and whatever files the project needs, free to download.">
 <link rel="canonical" href="https://builds.kestrelsnest.social/">
 <meta property="og:type" content="website">