Add strategy about setup sessions to Learners at Many Levels situation

### How could the content be improved?

One of the strategies missing from the list in the Sticky Situations - Learners at Many Levels exercise is having a session in advance of the workshop to help learners get their software installations done before the morning that the workshop begins. Being left behind right at the beginning because of an issue with bash / RSTudio / Jupyter is a disaster for a learner.

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

https://carpentries.github.io/instructor-training/instructor/21-management.html#sticky-situations-1-learners-at-many-levels

Open the linked Sticky Situations - Learners at Many Levels section in instructor/21-management.html and review how the existing strategies are presented. Add a strategy about holding a setup session before the workshop for software installation, covering the learner impact described in the issue; done means the strategy appears clearly in that section.

Written by the indexing model from the issue text.