[Sets/Cater-Waiter]: Rework Concept Docs (#3456)

* Reworked and shortened concept and introduction docs.
* Added Exercism note about symmetric difference to `about.md`.
* Final intro touchups.
* Further touchups to terminal examples.  Removed iPython refs.
* Update concepts/sets/about.md
* Slight wording change to one test
[no important files changed]

Author: BethanyG