Only publish docs on main

Author: dstansby

Diff for: .github/workflows/docs.yml

@@ -40,6 +40,7 @@ jobs:
     name: Upload docs to GitHub pages
     runs-on: ubuntu-latest
     needs: build-docs
+    if: github.event_name == 'push' && github.ref_name == 'main'
     steps:
     - uses: actions/checkout@v3
     - uses: actions/download-artifact@v3