docs: add vercel.json to fix Vercel deployment

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

docs/vercel.json

@@ -0,0 +1,6 @@
+{
+  "buildCommand": "pnpm docs:build",
+  "outputDirectory": ".vitepress/dist",
+  "installCommand": "pnpm install",
+  "framework": "vitepress"
+}