CI: add website build job using Node 22 (#110)

The Astro version used by the website requires Node.js >=22.12.0.
Add a dedicated `build-website` job to checks.yaml that runs
`npm ci` and `npm run build` inside the `website/` directory
with Node 22, keeping the existing library jobs on Node 20.

🤖 Generated with [eca](https://eca.dev)

Co-authored-by: eca <git@eca.dev>

Author: afucher

.github/workflows/checks.yaml

@@ -35,3 +35,15 @@ jobs:
           node-version: 20
       - run: npm ci
       - run: npm run build
+
+  build-website: # sanity check that the Astro website builds without errors
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+      - run: npm ci
+        working-directory: website
+      - run: npm run build
+        working-directory: website