chore: add package manager

Author: m-mdy-m

package.json

@@ -20,6 +20,7 @@
     "preview": "astro preview",
     "astro": "astro"
   },
+  "packageManager": "pnpm@9.15.4",
   "dependencies": {
     "@astrojs/starlight": "^0.32.5",
     "@astrojs/starlight-tailwind": "^3.0.1",