Add doc about triggering test workflow manually in fork repository

Author: ericpre

doc/devel/testing.rst

@@ -159,7 +159,9 @@ workflows
 GitHub Actions should be automatically enabled for your personal Matplotlib
 fork once the YAML workflow files are in it. It generally isn't necessary to
 look at these workflows, since any pull request submitted against the main
-Matplotlib repository will be tested.
+Matplotlib repository will be tested. The tests workflow is skipped in fork
+repositories but you can trigger a run manually from the `github web
+interface <https://docs.github.com/en/actions/managing-workflow-runs/manually-running-a-workflow>`_.
 
 You can see the GitHub Actions results at
 https://github.com/your_GitHub_user_name/matplotlib/actions -- here's `an