Abridged reference for Criterion

[hobovsky]

There's a stub of Criterion reference, but it needs to be improved.

Online documentation of Criterion is to be found here: https://criterion.readthedocs.io/en/master/intro.html

Codewars docs could use some small excerpt of assertions and control structures most commonly used in kata, and a link to full docs.

[hobovsky]

Hinted by @ggorlen here: https://github.com/codewars/docs/issues/207#issuecomment-758143049

There's a small walkthrough on improving criterion assertion messages here which may be useful.