Concept Regular Expressions: Add some more details

There are some ideas for more details to be added in the regular expressions concept, especially in the about.md file. See https://github.com/exercism/go/issues/2037#issuecomment-1105809316 (and the original issue there) for details.

Only topics that are relevant for solving the exercise or for the basic understanding should be added to the introduction.md of the concept. Everything else should only be part of the about.md file.

If you make an addition to the concept introduction.md, the same change should be made to the introduction.md file of the "animal magic" exercise.

General documentation:
- [Concept Exercises](https://exercism.org/docs/building/tracks/concept-exercises)
- [Concepts](https://exercism.org/docs/building/tracks/concepts)