Only publish documentation from the main branch

ref: #39

Author: has2k1

.github/workflows/docs.yml

@@ -31,6 +31,7 @@ jobs:
           name: docs-html
           path: docs/_build/html/
       - name: publish docs
+        if: github.ref_name == 'main' && github.ref_type == 'branch'
         uses: peaceiris/actions-gh-pages@v3
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}