Firebase runner is not documented (or at least not easy to find in the documentation)

[musslick]

The firebase_runner that we use in the closed-loop example is not documented. At least, I could not easily find it in the documentation, only the firebase_prolific_runner. We should add corresponding documentation or re-organize the existing one.

A search for firebase_runner links to an orphant tutorial. That's the only place where the runner is mentioned.