Adding concurrency concepts to the syllabus

This is a placeholder issue for the work on concurrency related concepts and learning exercises.

## Overview
This will be the first bit for teaching concurrency in the syllabus. Overall, we want to add the following concepts:

- goroutines and waitgroups (two concepts in one exercise to allow for easier testing)
- channels (potentially incl. range over channel)
- select
- mutex

Later we could add more things:
- atomics
- context
- errgroup, syncmap and other helper stuff

Besides that, we also already have some unfinished work regarding net/http in store.

## Important Docs

If you have not yet contributed to concept exercises before, the followig upfront reading is required to acquire the necessary background knowledge.

Here you can read about what Concept Exercises are and how they are structured:

- [What are Concept Exercises?](https://exercism.org/docs/building/product/concept-exercises)
- [Concept Exercises](https://exercism.org/docs/building/tracks/concept-exercises)
- [Concepts](https://exercism.org/docs/building/tracks/concepts)

Also, be aware of these general guidelines:

- [Exercism Markdown Specification](https://exercism.org/docs/building/markdown/markdown)
- [Exercism Formatting and Style Guide](https://exercism.org/docs/building/markdown/style-guide)