[DOCS] for sample: python callable functions

### Which sample?

https://github.com/firebase/functions-samples/blob/main/Python/quickstarts/callable-functions/README.md

### What is the issue with this sample's docs?

README is empty (unlike its node counterpart). Also, no guidance is provided on how to test a python callable function (testing with the cloud console UI did not work for me, probably because I had no guidance). Does the cloud console test not work with callable functions? If so, this should be stated explicitly along with the advised method for testing.