Update gh-pages deployment that uses a deprecated node.js (#72)

Author: ceriottm

.github/workflows/docs.yml

@@ -97,7 +97,7 @@ jobs:
 
       - name: deploy to gh-pages
         if: github.event_name == 'push'
-        uses: peaceiris/actions-gh-pages@v3
+        uses: peaceiris/actions-gh-pages@v4
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: ./gh-pages/