Add blurb to Concept Exercises

Each Concept will have to define a _blurb_, which is a short description of the exercise.

In an earlier PR, a placeholder blurb has been added to the .meta/config.json file of each Concept Exercise. The placeholder blurbs should be replaced with an actual blurb for each Concept Exercise. Blurbs must be limited to 350 chars and will be truncated in some views.

For forked exercises, it might be useful to check how other tracks have defined their blurb.

The blurb will be displayed on a track's exercises page and on exercise tooltips. For example:

See the [Concept Exercise spec](https://github.com/exercism/docs/blob/main/anatomy/tracks/concept-exercises.md#file-metaconfigjson) for more information.

## Tracking

https://github.com/exercism/v3-launch/issues/23

Concept Exercise specを確認し、各Concept Exerciseの.meta/config.jsonファイルを調査します。各プレースホルダーの概要文を、350文字以内のそのExercise専用の説明に置き換えます。forkされたExerciseについては、他のtrackの概要文を参考にします。すべてのConcept Exerciseに、Exercisesページとツールチップに適した完成済みの概要文があれば完了です。

索引モデルが issue の本文から書いたものです。