Infra: Only add preview links for PRs to python/peps (#4399)

Co-authored-by: Hugo van Kemenade <[email protected]>

Author: henryiii

.github/workflows/documentation-links.yml

@@ -16,6 +16,7 @@ concurrency:
 jobs:
   documentation-links:
     runs-on: ubuntu-latest
+    if: github.event.repository.fork == false
     steps:
       - uses: readthedocs/actions/preview@v1
         with: