Primer box for compartmental model

Include an optional primer box (e.g. to break down components, and make it clear that dS/dt is the rate at which number of susceptible individuals changes over time, and can write this in terms of people who enter/leave the compartment)?

Suggested by @adamkucharski : https://github.com/epiverse-trace/tutorials-late/pull/51#discussion_r1826602783