CI: fix docs-deploy Node version for Astro build (#111)

The deploy workflow ran the library build (webpack, Node 20) and the
Astro website build in the same job under node-version 20, causing
Astro to fail with "Node.js v20 is not supported".

A single job can only run one Node version, so split the build job:

- build-library (Node 20): builds the webpack bundle and uploads it
  as a GitHub Actions artifact
- build-website (Node 22): downloads the bundle, copies it into
  website/public/, installs website deps, builds the Astro site, and
  uploads the Pages artifact
- deploy: now depends on build-website (unchanged otherwise)

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

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

Author: afucher

.github/workflows/docs-deploy.yml

@@ -19,7 +19,7 @@ concurrency:
   cancel-in-progress: false
 
 jobs:
-  build:
+  build-library:
     runs-on: ubuntu-22.04
     steps:
       - name: Checkout
@@ -36,6 +36,30 @@ jobs:
       - name: Build library
         run: npm run build
 
+      - name: Upload library bundle
+        uses: actions/upload-artifact@v4
+        with:
+          name: astrochart-bundle
+          path: dist/astrochart.js
+
+  build-website:
+    needs: build-library
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 22
+
+      - name: Download library bundle
+        uses: actions/download-artifact@v4
+        with:
+          name: astrochart-bundle
+          path: dist
+
       - name: Copy bundle to website
         run: cp dist/astrochart.js website/public/astrochart.js
 
@@ -53,7 +77,7 @@ jobs:
           path: website/dist
 
   deploy:
-    needs: build
+    needs: build-website
     runs-on: ubuntu-22.04
     environment:
       name: github-pages