[v3] Extract Concepts from v2 exercise: nucleotide-count

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

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

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

v2 の `nucleotide-count` exercise を読み、リンクされているコミュニティソリューションを確認して、必要なアプローチと概念を特定します。`languages/cpp/reference/exercise-concepts/nucleotide-count.md` を作成し、それぞれの概念がなぜ必要なのかを説明します。関連するアプローチに含まれるすべての概念の一覧が文書に記載されていれば完了です。

索引モデルが issue の本文から書いたものです。