[v3] Extract Concepts from v2 exercise: nth-prime

Extract the list of concepts required to solve the v2 C++ exercise: `nth-prime`. 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/cpp/exercises/nth-prime/solutions`). The goal of this task is to help determine the total list of concepts that make up `nth-prime`.

The list of concepts should be stored in the following document: `languages/cpp/reference/exercise-concepts/nth-prime.md`. [Use this link to create a PR to add this document][pr-link].

[pr-link]: ../new/master?filename=languages/cpp/reference/exercise-concepts/nth-prime.md

从 v2 `nth-prime` 练习开始，查看链接的 C++ 社区解法，以确定多个方法中使用的概念。在 `languages/cpp/reference/exercise-concepts/nth-prime.md` 中记录每个必需的概念及其必要原因；完成的标准是文档涵盖完整的概念列表，但不实现该练习。

由索引模型根据 Issue 内容生成。