[v3] Implement new Concept Exercise: `Reduce`

This issue describes how to implement the `Reduce` concept exercise for the Clojure track.

## Getting started

**Please please please read the docs before starting.** Posting PRs without reading these docs will be a lot more frustrating for you during the review cycle, and exhaust Exercism's maintainers' time. So, before diving into the implementation, please read up on the following documents:

- [The features of v3](https://github.com/exercism/v3/blob/master/docs/features-of-v3.md).
- [Rationale for v3](https://github.com/exercism/v3/blob/master/docs/rationale-for-v3.md).
- [What are concept exercise and how they are structured?](https://github.com/exercism/v3/blob/master/docs/concept-exercises.md)

Please also watch the following video:

- [The Anatomy of a Concept Exercise](https://www.youtube.com/watch?v=gkbBqd7hPrA).

## Goal

The goal of this exercise is to teach the student how the `reduce` function is implemented in Clojure.

## Learning objectives

The student will gain a general understanding of:

- The concept of reducing a function on a collection

## Out of scope

- Transducers

## Concepts

- `higher-order-functions`

## Prerequisites

- `basics`

## Resources to refer to

[Clojure for the Brave and True - Core Functions in Depth](https://www.braveclojure.com/core-functions-in-depth/)

## Representer

The [Clojure Representer](https://github.com/exercism/clojure-representer) is currently being implemented and contributions are welcome.

## Analyzer

The [Clojure Analyzer](https://github.com/exercism/clojure-analyzer) is currently being implemented and contributions are welcome.

## Implementing

To implement a concept exercise, the following files must be created:

```
languages
└── clojure
└── exercises
└── concept
└──
├── .docs
| ├── instructions.md
| ├── introduction.md
| ├── hints.md
| └── after.md (optional)
├── .meta
| |── design.md
| └── Example.clj
├── .clj
└── _test.clj
```

## Help

If you have any questions while implementing the exercise, please post the questions as comments in this issue.

Lies docs/features-of-v3.md, docs/rationale-for-v3.md und docs/concept-exercises.md und sieh dir anschließend das verlinkte Video Anatomy of a Concept Exercise an. Erstelle die aufgelisteten Dateien unter languages/clojure/exercises/concept/, einschließlich der Dokumentations-, Design-, Beispiel-, Quellcode- und Testdateien. Die Aufgabe ist abgeschlossen, wenn die Übung das Reduzieren einer Funktion auf einer Collection vermittelt, ohne Transducers abzudecken, und den festgelegten Voraussetzungen und dem Konzept folgt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.