Support litedown rendering

[jeroen]

Seems like we can't render litedown Rmd with knitr::rmarkdown

Original: https://cran.r-project.org/web/packages/litedown/vignettes/markdown-output.html
R-universe: https://yihui.r-universe.dev/articles/litedown/markdown-output.html

[jeroen]

r-universe-org/build-source@52f61c3

[TimTaylor]

I'm sure you've noticed but just mentioning in case.... it seems that if images are present the output is event more mangled (cutting everything off after half an image 🤷‍♂️ ), e.g.

Original: https://cran.r-project.org/web/packages/knitr/vignettes/knitr-html.html
R-universe: https://yihui.r-universe.dev/articles/knitr/knitr-html.html

[jeroen]

It seems that customizing the default styling of litedown vignettes is actually quite complicated, because of the many css/js plugins that users can specify in the fontmatter. Those plugins seems to conflict with e.g. bootstrap.

For now I have a temporary solution where vignettes without any plugins use a custom template. Other vignettes we use basically a copy of the default litedown template, but with a copy of our article.js such that it can be viewed in the iframe.