Bump peaceiris/actions-gh-pages to 3.9.0. (#63)

Author: felixfontein

.github/workflows/_shared-docs-build-publish-gh-pages.yml

@@ -91,9 +91,9 @@ jobs:
 
       - name: Publish
         # this action uses a token with contents:write, pinning to commit
-        # 068dc23d9710f1ba62e86896f84735d869951305 == v3.8.0
-        # https://github.com/peaceiris/actions-gh-pages/releases/tag/v3.8.0
-        uses: peaceiris/actions-gh-pages@068dc23d9710f1ba62e86896f84735d869951305
+        # de7ea6f8efb354206b205ef54722213d99067935 == v3.9.0
+        # https://github.com/peaceiris/actions-gh-pages/releases/tag/v3.9.0
+        uses: peaceiris/actions-gh-pages@de7ea6f8efb354206b205ef54722213d99067935
         with:
           github_token: ${{ secrets.GH_TOKEN }}
           publish_dir: html