Roxygen

[olivroy]

Closes #778. Part of #776

(the last commit didn't want to show up on the PR)

Imo, this simplifies the documentation by using markdown. Adding () to signify a function, to make it distinct from arguments.
roxygen2md::roxygen2md() + tweaks
Remove broken links
Simplify documentation for data https://r-pkgs.org/data.html#sec-documenting-data

A round of doc improvement could probably be done prior to release.

as an example, see how on the tmap website, the group argument doc is not rendered correctly, without Roxygen(markdown = TRUE)

[mtennekes]

Excellent, thx! There is one small merging conflict. Which one shall we use @olivroy ?

[mtennekes]

Oh, and when you are still working on DESCRIPTION @olivroy , please don't forget to put your name on the @authors@R list :-) (fully deserved)

[olivroy]

@mtennekes Merged conflict + added myself as ctb while I was at it.

Thanks!