ci: don't add doc links to fork's PRs

Signed-off-by: Henry Schreiner <[email protected]>

Author: henryiii

.github/workflows/documentation-links.yml

@@ -16,6 +16,7 @@ concurrency:
 jobs:
   documentation-links:
     runs-on: ubuntu-latest
+    if: github.event.pull_request.head.repo.full_name == 'python/peps'
     steps:
       - uses: readthedocs/actions/preview@v1
         with: