Add exercise notebook dev to contributing

We should probably include a note about how exercise notebooks are developed in `CONTRIBUTING.md`, lest people think that they should edit those files directly.

A lot of this is now covered in the readme for the `exercise-notebook-answers` repo, but the highlights, including the fact that that repo will delete exercise notebooks that do not exist there.

Start with CONTRIBUTING.md and compare its exercise-notebook guidance with the README in the exercise-notebook-answers repository. Add a concise note explaining how exercise notebooks are developed, including that notebooks missing from that repository may be deleted. Done means contributors are directed away from editing those files directly.

Written by the indexing model from the issue text.