Add pkgdown reference index sections

[Bisaloo]

sivirep currently provides many utility function to manipulate data provided by the SIVIGILA source.

As a result, the pkgdown reference index can be a bit overwhelming. It would be good to group the functions into logical sections, as ggplot2 for example is doing.

Your good design work makes it particularly easy here since you have use consistent function prefixes and names moulds throughout the package so you can just use selectors such as starts_with().

Related pkgdown documentation: https://pkgdown.r-lib.org/reference/build_reference.html#reference-index