Evaluate intended audience for session plans

[adamblanchard]

There's some confusion and lack of consistency on who we write the session plans for.

We originally concluded (small discussion here) to write them for the mentor.

We recently discussed this a bunch on this pr, since even though I wrote it with the intended audience of the mentor, i still wrote some of the content as if I was talking to trainees. I saw it as a kind of "script" that mentors could use. But this causes some confusion in the writing.

This task should:

1. Clarify the actual intended audience
2. Tidy up at least the node module (linked above)
3. Share the decision with the rest of the curriculum crew group / document it somewhere
4. Evaluate other modules and update wording if needed