Add a GitHub workflow to deploy gh-pages to GitHub Pages

When merging PRs, or pushing changes directly, we will want to deploy
the result to GitHub Pages. That's this new workflow's job.

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

Author: dscho

.github/actions/deploy-to-github-pages/action.yml

@@ -16,9 +16,11 @@ outputs:
 runs:
   using: "composite"
   steps:
-    - name: push changes
+    - name: push changes (if needed)
       shell: bash
-      run: git push origin HEAD
+      run: |
+        test "$(git rev-parse HEAD)" = "$(git rev-parse refs/remotes/origin/${{ github.ref_name }})" ||
+        git push origin HEAD:${{ github.ref }}
 
     - name: un-sparse worktree to prepare for deployment
       shell: bash

.github/workflows/deploy.yml

@@ -0,0 +1,22 @@
+name: Deploy to GitHub Pages
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - gh-pages
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    permissions:
+      pages: write # to deploy to GitHub Pages
+      id-token: write # to verify that the deployment source is legit
+    environment:
+      name: github-pages
+      url: ${{ steps.deploy.outputs.url }}
+    steps:
+      - uses: actions/checkout@v4
+      - name: deploy to GitHub Pages
+        id: deploy
+        uses: ./.github/actions/deploy-to-github-pages