Demos: emailing host to request lesson/episode not on official list

[tobyhodges]

In #1545, @sstevens2 asked whether trainees should contact the teaching demo session host directly to discuss when they want to do their demo with a lesson or episode not listed or listed as ineligible.

@karenword responded:

This is something we've gone back and forth with. I think some Trainers are comfortable making this determination but it can cause stress, especially for new Trainers who aren't sure how they should handle these requests. If we have them email the session host, we should provide corresponding guidelines for session hosts (e.g in the handbook).

I am opening this issue as a place for that to be discussed, so you can decide whether to change the guidance here and add the relevant content to https://docs.carpentries.org.