Make vignette for plots

[thibautjombart]

Plots are a huge aspect of epidemic curves, and it would be useful to provide more details on how nice plots can be generated with incidence objects. Version < 2 had a dedicated vignette, which we could re-use and update. In particular:

• [ ] clarify how different date indices versus groups are represented
• [ ] customize x/y axes
• [ ] use custom color scales and colors
• [ ] use facet plots
• [ ] customize facets