How-to Docs: Creating a new concept exercise

We have the "spec" style documentation about what is in a concept exercise once it is done but we don't have a document about how to approach creating a new exercise from scratch.

Some notes on what could be mentioned:
- Start from thinking about learning objectives (i.e. what should the student learn from the exercise) if they are not yet mentioned in the issue already. If the concept exists already, check what is taught there so you know what the exercise should practice.
- If you want to save some time, check whether there is some existing concept exercise from another track that has a similar concept. A list can be found here: https://exercism.org/docs/building/tracks/stories. When adapting an existing exercise, make sure all tasks actually make sense for your track. Modify the exercise to the needs of your language.
- (From @kytrinyx:) When inventing an exercise from scratch, think about the expressions/syntax that you would like to see the student write as part of the solution (based on the learning objectives). Then try to retro-fit some story onto those things.
- Most stories/themes can be used for any kind of exercise, if you struggle to come up with a story, read some of the existing stories (https://exercism.org/docs/building/tracks/stories). Often, that can help to spark some inspiration even if there is no 1to1 match.
- The introduction can be taken from the concept introduction if the concept exists already (e.g. via configlet generate), usually no additional content has to be written in this case.
- Copy some existing exercise so you get all the right general files set up, potentially already remove existing content and add todo markers instead.
- Possible order of approaching things: design.md (to clarify learning objectives), exemplar and instructions, tests, stub, configs