General improvements to COVID-19 data raw

[jamesmbaazam]

• Please add comment explaining the link and ideally a reference, see similar in {cfr};
• Explain the data preparation briefly;
• Ideally check whether the data author is okay with this data being provided, and possibly invite them to be a package contrib
• If this is simply the daily incidence data, prefer a source such as WHO Covid data, or {covidregionaldata} (with citation/explanation, see {cfr})

_Originally posted by @pratikunterwegs in https://github.com/epiverse-trace/epichains/pull/122#discussion_r1420333822_