General documentation about practice exercises

[junedev]

It came up in exercism/problem-specifications#2149 (review) and other conversations that we are currently missing documentation/explanation/rationale on practice exercises in general, e.g.

What makes a good practice exercise? It what ways should they challenge the student? Do we want to leave some ambiguity in the descriptions to make it more like real life or should descriptions always be as clear as possible? What are the criteria for adding a new exercise to problem spec or a track?

The above applies to the maintainer side. Additionally, we might also miss more documentation/tutorials around how to deal with practice exercises as a student (e.g. TDD, you need to select the concept to use on your own, potentially try different ones, etc.).

Sidenote: For learning exercises there is a bit more written down I think but there might be more needed as well.