deploy action should only run on the main branch

And should be updated to vefsion 4 to work with new GH security changes

.github/workflows/main.yml

@@ -39,7 +39,8 @@ jobs:
           opam exec -- dune build @doc
 
       - name: Deploy
-        uses: peaceiris/actions-gh-pages@v3
+        uses: peaceiris/actions-gh-pages@v4
+        if: github.ref == 'refs/heads/main'
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: _build/default/_doc/_html