Add some prep material about setTimeout

[illicitonion]

Some of the backlog issues in https://programming.codeyourfuture.io/data-groups/sprints/3/backlog/ require using setTimeout or setInterval.

We currently introduce setTimeout later on in a very full sprint teaching general asynchrony and promises https://programming.codeyourfuture.io/data-flows/sprints/3/prep/

I think we should add a block to https://programming.codeyourfuture.io/data-groups/sprints/3/prep/ introducing setTimeout (as a "fire and forget" form of asynchrony, where we don't worry about returning results).

This will:

1. Help with the coursework that sprint.
2. Give our learners more experience of passing callbacks around. (They will get confused and stuck on this)
3. Reduce the cognitive load of new concepts in Data Flows Sprint 3 where we're explaining way too many new concepts at once.