chore(deploy): add vercel.json for docs-site and playground

Encode the monorepo-aware build for each site so Git integration (or `vercel`
CLI) is turnkey: install + build the core at the repo root first, then the site.
Set each Vercel project's Root Directory to the matching subdir.

- docs-site:  cd .. && npm run docs:build      → .vitepress/dist
- playground: cd .. && npm run playground:build → dist

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ChanMeng666

docs-site/vercel.json

@@ -0,0 +1,7 @@
+{
+  "$schema": "https://openapi.vercel.sh/vercel.json",
+  "framework": null,
+  "installCommand": "cd .. && npm install",
+  "buildCommand": "cd .. && npm run docs:build",
+  "outputDirectory": ".vitepress/dist"
+}

playground/vercel.json

@@ -0,0 +1,7 @@
+{
+  "$schema": "https://openapi.vercel.sh/vercel.json",
+  "framework": null,
+  "installCommand": "cd .. && npm install",
+  "buildCommand": "cd .. && npm run playground:build",
+  "outputDirectory": "dist"
+}