Port docs on design of concepts & concept exercises from v3 repo

[SaschaMann]

The v3 docs contained information on the design of concepts and concept exercises that went beyond the technical spec: https://github.com/exercism/v3/blob/main/docs/concept-exercises.md#how-are-concept-exercises-designed-and-structured

These are currently not available in /docs but are quite useful for people who aren't intimately familiar with the ideas behind v3.

(Mostly creating this issue to make the old docs findable again, @iHiD is already aware that they haven't been ported)