Write solutions to open ended notebook exercises

One workshop participant noted that they would like to check their "answers" to some of the open ended notebook exercises against a "solution". We think this is mainly an issue for the specutils notebooks. We need an instructor solution version for participants to look at.

Start by inventorying the specutils notebooks and identifying which exercises are open ended and need instructor solutions. Review the existing workshop notebook structure, then provide solution versions for the identified exercises; the work is done when participants can compare their answers against those solutions.

Written by the indexing model from the issue text.