Make pkgdown articles available as vignettes

[GregorDeCillia]

It might be a nice addition to make the current pkgdown articles (or some of them) also available as vignettes so that they can be used offline. Currently, the articles use some customizations (in vignettes/R/) that might make this not 100% straight forward

Persumably, the tooltips should be disabled in the offline version because tippy.js is currently loaded via a CDN. Dependencies to {fansi} should also be avoided and possibly replaced with cli::ansi_html().