Add a GitHub workflow to deploy the site

Technically, we could use the default strategy, which does not require
any confiuration.

However, the default strategy has 3 jobs, downloads a hefty Docker
container containing Jekyll and whatnot, and takes over half a minute
to deploy two files (okay, three, but the node.js script is not needed).

Let's instead adapt the simple and fast GitHub starter workflow from
https://github.com/actions/starter-workflows/blob/main/pages/static.yml

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

.github/workflows/gh-pages.yml

@@ -0,0 +1,37 @@
+name: Deploy GitHub Pages
+
+on:
+  push:
+    branches: [gh-pages]
+  workflow_dispatch:
+
+# Sets permissions of the GITHUB_TOKEN to allow deployment to GitHub Pages
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+# Allow only one concurrent deployment, skipping runs queued between the run in-progress and latest queued.
+# However, do NOT cancel in-progress runs as we want to allow these production deployments to complete.
+concurrency:
+  group: "pages"
+  cancel-in-progress: false
+
+jobs:
+  deploy:
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Setup Pages
+        uses: actions/configure-pages@v5
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: '.'
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4