fix(ci): pin Node 22 for the Astro build (Astro 6 requires >=22.12.0)

withastro/action defaults to Node 20, which Astro 6 rejects ("Node.js v20 is not
supported"). Set node-version: 22 so `astro build` runs on a supported runtime.

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

Author: lidge-jun

.github/workflows/deploy-docs.yml

@@ -31,6 +31,7 @@ jobs:
         uses: withastro/action@v3
         with:
           path: ./docs-site
+          node-version: 22 # Astro 6 requires Node >= 22.12.0 (the action defaults to 20)
           package-manager: bun@latest
 
   deploy: