feat(site): robots, sitemap reference, vercel redirects + cache headers

site/public/robots.txt

@@ -0,0 +1,4 @@
+User-agent: *
+Allow: /
+
+Sitemap: https://termscp.rs/sitemap-index.xml

site/vercel.json

@@ -0,0 +1,22 @@
+{
+  "$schema": "https://openapi.vercel.sh/vercel.json",
+  "redirects": [
+    { "source": "/index.html", "destination": "/", "permanent": true },
+    { "source": "/get-started.html", "destination": "/install", "permanent": true },
+    { "source": "/get-started", "destination": "/install", "permanent": true },
+    { "source": "/updates.html", "destination": "/install", "permanent": true },
+    { "source": "/user-manual.html", "destination": "https://docs.termscp.rs", "permanent": true },
+    { "source": "/user-manual", "destination": "https://docs.termscp.rs", "permanent": true },
+    { "source": "/changelog.html", "destination": "https://github.com/veeso/termscp/blob/main/CHANGELOG.md", "permanent": true }
+  ],
+  "headers": [
+    {
+      "source": "/fonts/(.*)",
+      "headers": [{ "key": "Cache-Control", "value": "public, max-age=31536000, immutable" }]
+    },
+    {
+      "source": "/install.sh",
+      "headers": [{ "key": "Content-Type", "value": "text/x-shellscript; charset=utf-8" }]
+    }
+  ]
+}