chore: add note to `CONTRIBUTING.md` about how to run tests against Connect

We removed the `test-connect` jobs (#727) from this repository and adjusted the functionality of the `rsconnect-python-tests-at-night` in Connect so that they can be run against a specific branch of `rsconnect-python` via workflow dispatch. 

We need to document that these tests should be triggered on the Connect side for each new PR in `rsconnect-python` and give instructions for how to do so.