Add a figure of delay distributions in a vignette

[avallecam]

Is your feature request related to a problem? Please describe.

The definitions of all the delay distributions in the drafted vignette are clear and concise. I usually forget their precise definition and this text is helpful in that matter. However, I often need to check summary figures to complement them.

https://github.com/epiverse-trace/epiparameter/blob/ed0da13a8802e364c1e35fff97323e319838a07b/vignettes/epiparameter.Rmd#L32-L51

Describe the solution you'd like

The figures that I usually use as visual reminders are these:

• The relationship of incubation period and serial interval
• Figure 1 of the demonstrative timeline of a transmission chain for a pair of infector and infected

But probably there are other ones more concise for this issue.

A graphical complement to the package can add an additional utility to the package website for users:

• a descriptive and visual reminder of definitions (as my bookmarked links above) and
• a constant reminder to use them with the package!