don't try running doc site deploy on PRs

Author: martinfouilleul

.github/workflows/build-doc-site.yml

@@ -34,6 +34,7 @@ jobs:
           path: "./doc/mkdocs/site"
 
   deploy:
+    if: ${{ github.event_name == 'push' }}
     runs-on: ubuntu-latest
     needs: build