[v3] Extract Concepts from v2 exercise: atbash-cipher

[aimorris]

Extract the list of concepts required to solve the v2 purescript exercise: atbash-cipher. For each concept, describe why it is required to solve the exercise. Keep in mind that an exercise can have multiple approaches (you can find community solutions here: https://exercism.io/tracks/purescript/exercises/atbash-cipher/solutions). The goal of this task is to help determine the total list of concepts that make up atbash-cipher.

The list of concepts should be stored in the following document: languages/purescript/reference/exercise-concepts/atbash-cipher.md. Use this link to create a PR to add this document.