Flesh out description of poker exercise

The `poker` exercise currently only links to a wikipedia article for an explanation of the rules.
This is problematic in a couple of ways:

- The wikipedia article mentions some special rules, which we ignore
- If you don't have internet once you've downloaded the exercise, it is not easy to work on the exercise

I propose we do the same we did for `yacht`, another card game exercise, by fleshing out the description: https://github.com/exercism/problem-specifications/blob/main/exercises/yacht/description.md