Bump peaceiris/actions-gh-pages from 3 to 4

Bumps [peaceiris/actions-gh-pages](https://github.com/peaceiris/actions-gh-pages) from 3 to 4.
- [Release notes](https://github.com/peaceiris/actions-gh-pages/releases)
- [Changelog](https://github.com/peaceiris/actions-gh-pages/blob/main/CHANGELOG.md)
- [Commits](peaceiris/actions-gh-pages@v3...v4)

---
updated-dependencies:
- dependency-name: peaceiris/actions-gh-pages
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/docs.yml

@@ -67,7 +67,7 @@ jobs:
           TZ=UTC make -C docs html
 
       - name: Deploy (to gh-pages branch) only on demand
-        uses: peaceiris/actions-gh-pages@v3
+        uses: peaceiris/actions-gh-pages@v4
         if: ${{ github.event.inputs.deploy }}
         with:
           publish_branch: gh-pages