Add image demonstrating meaning of "mirror learner's environment"

### How could the content be improved?

Per discussion on #1043 it would be ideal to have a persistent (e.g. learners can review after the training) example of what it looks like to mirror or not the learner's environment.

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

https://carpentries.github.io/instructor-training/instructor/17-live.html

Start with the instructor-training lesson at https://carpentries.github.io/instructor-training/instructor/17-live.html and review the discussion linked from issue #1043. Add a persistent example image that makes the difference between mirroring and not mirroring the learner's environment clear, then verify that learners can review it from the lesson.

Written by the indexing model from the issue text.