Make vignette for alternative date groupings

[thibautjombart]

This was present in version <2, and would probably be useful to keep. My understanding is that the new version integrates seemlessly with as_period and other date groupings, so this might be even simpler than before?

[TimTaylor]

If we go with version2 there is no need for an additional vignette as there is no concept of a date grouping native to incidence2.