Add data for examples to package

[pratikunterwegs]

This issue is to request that Covid data used in the examples should be included in the package, rather than being downloaded via an API or taken from a data package. The data should also be used in the package vignettes and function examples.

This stems from the issue that these packages are either not on CRAN, or not actively developed, or both. This is the case for {covidregionaldata} and {owidR}. {owidR} was on CRAN but has been removed as of 8th August 2023. This reopens the issue raised in #61.