Explore {litedown} for rendering vignettes

[MichaelChirico]

{knitr} requires R>=3.6.0; that means we can't render vignettes on our oldest supported versions of R (3.3, 3.4, 3.5).

@jangorecki sugegsts trying to render them instead with {litedown}:

d66e12b#r145771409

https://cran.r-project.org/web/packages/litedown/index.html

https://github.com/yihui/litedown

{litedown} is only 5 months old:

yihui/litedown@03db997

So we can expect to be among the first "major" users of the package if we make the switch -- I expect that to generate a decent amount of work filing/even fixing issues with upstream.

@jangorecki do you want to take the lead on owning the potential switch (exploring/validating/maintaining)?

[jangorecki]

IMO we could as well move away from pkgdown to new yihui/litedown#24 (comment)
Pkgdown was causing extra burden with dependencies (new and many OS deps) and was at some point cached in a docker image, so it is not even updated in our CI for a longer while now.

[ben-schwen]

IMO we could as well move away from pkgdown to new yihui/litedown#24 (comment) Pkgdown was causing extra burden with dependencies (new and many OS deps) and was at some point cached in a docker image, so it is not even updated in our CI for a longer while now.

I would keep this as an extra issue, one step at a time!