Update GitHub Pages deployment configuration

Author: Atsushi Ambo

.github/workflows/main.yml

@@ -78,3 +78,5 @@ jobs:
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: ./site
+          destination: ./  # Deploy to the root of gh-pages branch
+          keep_files: true  # Keep existing files in the gh-pages branch