Bump peaceiris/actions-gh-pages from 3.8.0 to 3.9.0

Bumps [peaceiris/actions-gh-pages](https://github.com/peaceiris/actions-gh-pages) from 3.8.0 to 3.9.0.
- [Release notes](https://github.com/peaceiris/actions-gh-pages/releases)
- [Changelog](https://github.com/peaceiris/actions-gh-pages/blob/main/CHANGELOG.md)
- [Commits](068dc23d97...de7ea6f8ef)

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

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

.github/workflows/docs.yaml

@@ -22,7 +22,7 @@ jobs:
         run: mdbook build
           
       - name: Deploy to GitHub Pages
-        uses: peaceiris/actions-gh-pages@068dc23d9710f1ba62e86896f84735d869951305 # v3.8.0
+        uses: peaceiris/actions-gh-pages@de7ea6f8efb354206b205ef54722213d99067935 # v3.8.0
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: ./target/book