ci: add GitHub Pages demo site deployment workflow

Author: armstrongl

.github/workflows/demo.yml

@@ -0,0 +1,54 @@
+name: Deploy Demo
+
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: pages
+  cancel-in-progress: true
+
+jobs:
+  build:
+    name: Build demo site
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+      - name: Install Hugo
+        uses: peaceiris/actions-hugo@v3
+        with:
+          hugo-version: "0.145.0"
+          extended: true
+      - name: Build rulebound
+        run: go build -o rulebound .
+      - name: Create Pages config
+        run: |
+          cp examples/starter-package/rulebound.yml /tmp/rulebound-demo.yml
+          sed -i 's|baseURL: /|baseURL: /rulebound/|' /tmp/rulebound-demo.yml
+      - name: Generate demo site
+        run: ./rulebound build examples/starter-package --output ./demo-output --config /tmp/rulebound-demo.yml
+      - name: Upload Pages artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: ./demo-output
+
+  deploy:
+    name: Deploy to GitHub Pages
+    needs: build
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4