upload actual book folder that contains static html build

Author: mmsaki

.github/workflows/docs.yml

@@ -44,6 +44,6 @@ jobs:
         uses: peaceiris/actions-gh-pages@v3
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
-          publish_dir: ./docs
+          publish_dir: ./docs/book
           publish_branch: gh-pages
           force_orphan: true