Add GHA workflow to build site for GitHub Pages

.github/workflows/build.yml

@@ -0,0 +1,52 @@
+name: Build and deploy
+
+on:
+  push:
+    branches: [main, webr-0-3-0]
+  pull_request:
+    branches: [main]
+  release:
+    types: [published]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+
+      - name: Set up Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: '3.11'
+
+      - name: Upgrade pip
+        run: python -m pip install --upgrade pip
+
+      - name: Build jupyterlite
+        run: |
+          python -m pip install -e ".[dev]"
+          jupyter lite build
+
+      - name: Upload _output/ artifact
+        if: github.ref == 'refs/heads/main' || github.ref == 'refs/heads/webr-0-3-0'
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: "_output/"
+
+  deploy_gh_pages:
+    if: github.ref == 'refs/heads/main' || github.ref == 'refs/heads/webr-0-3-0'
+    needs: build
+
+    permissions:
+      pages: write
+      id-token: write
+
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+
+    runs-on: ubuntu-latest
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v1