[v3] Extract Concepts from v2 exercise: diamond

[declarationperfume]

Extract the list of concepts required to solve the v2 PureScript exercise: diamond. 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/diamond/solutions). The goal of this task is to help determine the total list of concepts that make up diamond.

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