fix(ci): add Node.js 22 setup for Astro 6 docs build

Astro 6 requires Node.js >= 22.12.0. Bun bundles Node 20.x which
is too old. Add actions/setup-node@v4 step before the build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: coleam00

.github/workflows/deploy-docs.yml

@@ -20,6 +20,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
       - uses: oven-sh/setup-bun@v2
         with:
           bun-version: 1.3.11