Add suggestion about not putting instructor template into learner etherpad

### How could the content be improved?

In the last two workshops I've taught (and HT to @fishercera for the suggestion), we have had two etherpads going - one for the instructors, created from the [template](https://pad.carpentries.org/ttt-template) and one for the learners that starts out with only the welcome, sign-up, and CoC info (approx lines 1-66 from the template). Then, we have pasted information from the template into the learner etherpad as we go, rather than giving them all of the information in the template from the start.

Maybe this is common practice, but I think we should add this suggestion to the instructor notes, because giving the learners the whole template from the start is overwhelming.

We might also want to update point 2 in the template email that goes out to the instructors, so that instead of saying "You can create an Etherpad for your training using this template: https://pad.carpentries.org/ttt-template. This template includes all of the exercises and key-points of the lessons. Please feel free to customize your Etherpad as appropriate." it says something like "You can create two etherpads for your training - one for the instructors from and a blank one for your learners where you can copy and paste information and exercises as you go through the episodes". @sheraaronhurt

### Which part of the content does your suggestion apply to?

https://carpentries.github.io/instructor-training/instructor/instructor-notes.html

Start by locating the source for the instructor notes page and the template email referenced in the issue, then read the Etherpad guidance and email wording. Update the documentation to recommend separate instructor and learner Etherpads and staged copying of content, and verify that both the instructor notes and email convey the suggestion clearly.

Written by the indexing model from the issue text.