Add slide references to instructor notes

[KristinaRiemer]

Within instructor notes, add specific slide numbers to "From the slides" sections to link slides to notes explicitly.

[mine-cetinkaya-rundel]

Maybe another placeholder instead of slide numbers? Because the slide number will change if you add another slide and that could get hard to keep track of. How about "this comes after blah blah..."?

[KristinaRiemer]

We were also thinking about using slide headers as the slide references instead of numbers? It's unclear what would be best to link particular slides to particular instructor note sections.