Document / reconsider `topics` field in track config

[senekor]

The field has no purpose from Exercism's viewpoint. Consider deleting it or documenting its rust-track-specific purpose. Probably in the track config data structure of the rust-tooling crate, where it could be an optional field with appropriate documentation.

---

Why is the topics key added? It is only really there for inspiration for the practices/prerequisites fields, but it serves no purpose from Exercism's viewpoint.

Originally posted by @ErikSchierboom in #1722 (comment)

[senekor]

Found the documentation, it's in this issue.

It seems like the purpose this field serves right now is that for any future work on the syllabus, these topics might help guide which exercise should be marked to practice which topic. One of the topics is "goofy_bowling_logic" so that list is definitely a draft that may support, not a well-crafted list of concepts ready to use.