docs: add vercel.json to fix Vercel deployment

Add engines field to package.json requiring Node >= 20.
Vercel project settings also need Node.js version updated from 18.x.

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Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

Author: 3 people

docs/package.json

@@ -12,5 +12,8 @@
     "vitepress": "1.6.4",
     "vue": "^3.5.0"
   },
-  "type": "module"
+  "type": "module",
+  "engines": {
+    "node": ">=20"
+  }
 }