covidregionaldata: Expand plotting and data summary tools + documentation

[mjrolland]

covidregionaldata is an R package that is used to provide subregional data for Rt estimates produced each day (https://epiforecasts.io/covid/). Due to developer time being limited, and the majority of the current focus being on EpiNow2, covidregionaldata is lacking on going support.

covidregionaldata currently provides data but little else. This is partly by design but is also a limitation. Potentially adding support for basic plotting or easy linking to other packages to do this (i.e incidence2 for example) would potentially be a great benefit.This could be as simple as adding a vignette outlining how one could simply plot the data.

[impact: This package is used to support subregional reproduction number estimates that are used routinely by organisations such as the WHO and multiple governments. Providing reliable data is crucial to this pipeline. ]
[originally proposed by @seabbs]
[suggested repo: https://github.com/epiforecasts/covidregionaldata | @seabbs]
[additional notes: ]