Bump peaceiris/actions-gh-pages from 3.7.3 to 4.0.0

Bumps [peaceiris/actions-gh-pages](https://github.com/peaceiris/actions-gh-pages) from 3.7.3 to 4.0.0.
- [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@bbdfb20...4f9cc66)

---
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/publish-docs.yml

@@ -92,7 +92,7 @@ jobs:
     - name: Deploy documentation to blueskyproject.io.
       # We pin to the SHA, not the tag, for security reasons.
       # https://docs.github.com/en/free-pro-team@latest/actions/learn-github-actions/security-hardening-for-github-actions#using-third-party-actions
-      uses: peaceiris/actions-gh-pages@bbdfb200618d235585ad98e965f4aafc39b4c501  # v3.7.3
+      uses: peaceiris/actions-gh-pages@4f9cc6602d3f66b9c108549d475ec49e8ef4d45e  # v4.0.0
       if: ${{ github.event.inputs.deploy }}
       with:
         deploy_key: ${{ secrets.ACTIONS_DOCUMENTATION_DEPLOY_KEY }}