ci: Add workflow to deploy to GitHub Pages

While it is not necessarily useful to have these demos in their default state
published to the web, this workflow can be used by a learner who has customised
the demos in their own repo.

The workflow is designed to fail gracefully if GitHub Pages is not available
(which is typically the case in a private repo owned by a free account) or not
configured correctly.

It also only runs on releases or when manually triggered, to avoid running it
each time a learner pushes or merges to main.

https://phabricator.endlessm.com/T35699

Author: wjt

.github/workflows/github-pages.yml

@@ -0,0 +1,83 @@
+name: "Publish to GitHub Pages"
+on:
+  workflow_dispatch:
+  release:
+    types:
+      - published
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+env:
+  GODOT_VERSION: 4.3
+
+jobs:
+  check:
+    name: Check if GitHub Pages is enabled
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check
+        id: check
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          if gh api "repos/${{ github.repository }}/pages" | jq --exit-status '.build_type == "workflow"'
+          then
+              echo "enabled=true" >> "$GITHUB_OUTPUT"
+          else
+              echo "# Not published to GitHub Pages" >> "$GITHUB_STEP_SUMMARY"
+              echo "" >> "$GITHUB_STEP_SUMMARY"
+              echo -n "Check that Pages is enabled, with the source set to GitHub Actions, in the " >> "$GITHUB_STEP_SUMMARY"
+              echo "[repository settings](https://github.com/${{ github.repository }}/settings/pages)." >> "$GITHUB_STEP_SUMMARY"
+          fi
+    outputs:
+      enabled: ${{ steps.check.outputs.enabled }}
+
+  build:
+    name: Build for web
+    needs:
+      - check
+    if: ${{ needs.check.outputs.enabled }}
+    runs-on: ubuntu-latest
+    container:
+      image: barichello/godot-ci:4.3
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          lfs: true
+
+      - name: Set up export templates
+        run: |
+          mkdir -v -p ~/.local/share/godot/export_templates/
+          mv /root/.local/share/godot/export_templates/${GODOT_VERSION}.stable ~/.local/share/godot/export_templates/${GODOT_VERSION}.stable
+
+      - name: Web Build
+        run: |
+          mkdir -v -p build/web
+          godot --headless --verbose --export-release "Web" ./build/web/index.html
+
+      - name: Upload Artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          name: web
+          path: build/web
+
+  publish:
+    name: Publish to GitHub Pages
+    needs:
+      - build
+    runs-on: ubuntu-latest
+    steps:
+      - id: deploy
+        name: Deploy to GitHub Pages
+        uses: actions/deploy-pages@v4
+        with:
+          artifact_name: web
+
+      - name: Show URL in summary
+        if: ${{ steps.deploy.outcome == 'success' }}
+        run: |
+          echo "Game published to: <${{ steps.deploy.outputs.page_url }}>" >> "$GITHUB_STEP_SUMMARY"