Add exercise section times to exercises

### How could the content be improved?

It would be helpful to have estimates of the time to be spent in each subsection of multi-part exercises (like write in etherpad then discuss or think-pair-share) rather than just an overall estimate of time. It's easy to not plan enough time for the discuss/debrief section and spend more time than intended on an exercise. If this is distracting in the main body, it could also be added as instructor notes (though I don't think transparency is a problem here).

Review the multi-part exercises referenced in the issue and identify where their overall time estimates are currently shown. Add estimates for each subsection, including discussion or debrief portions, while keeping the presentation consistent across exercises; done means each relevant subsection has a usable time estimate.

Written by the indexing model from the issue text.