Document bisect_cmd

[encukou]

In 2017, Victor added a useful tool to find failing tests, bisect_cmd, and blogged about it. I wish I knew about it before!
IMO, the devguide should mention it.

[Dev-satish]

I want to take this project and make the necessary enhancements,
it would be great if someone could tell me that do i need to rewrite the entire section and then add it to devguide or should i just mention it in devguide with some info followed by a link to this.

[CAM-Gerlach]

I'd think just mention it in e.g. a new short subsection under Running tests; since it's part of CPython itself and a submodule of the publicly documented test module, perhaps it should be documented in the main CPython docs like the rest of the test module?