Identify, write, and mark potential deep dives in the guide

[wrtsprt]

Adding certain optional chapters for deep dives in between the normal guide would make for a flexible curriculum and IMO could be interesting to cater to the different interests of the students. To do this we should identify and clearly highlight those chapters, collapse them by default and make sure that the rest of the guide doesn't rely on the result.

Bootstrap would be something like that, end of part 1, though we kind of rely on it at the end of part 2.

Could this be potentially demotivating for a student, if the coach proposes to not do a deep dive chapter and move on with the guide?