dnd-char (concept exercise): replace with other concept exercise

[ErikSchierboom]

One of the wip concept exercises is dnd-char, which is actually a (renamed) practice exercise. As we do not allow practice exercises to be converted to concept exercise, it should be deprecated and replaced with another concept exercise.

The concepts it teaches are:

• kwdef,
• using-macros

The replacement exercise could maybe be similar-ish to the current exercise, in that it uses a struct and randomly generated values.