[FR] Add downlit support for github_document()

[cderv]

Following #1881, it would be interesting to add downlit auto-linking support for github_document() format.

For syntax highlighting, I don't think it would play well in a github markdown .md document.

[cderv]

Currently, downlit_md_path() will do the autolinking but also the highlighting. This will generate some html code for the highlighting that won't be useful for Github rendering.

However, we could do it probably as it does not harm to have this html element - it will just clutter the diff in Github on the README. A proof of concept has been done already with this example here https://github.com/r-prof/procmaps/blob/main/README.md from initial PR r-lib/downlit#43 by @krlmlr
Autolinking is done and correctly works on Github. However if we look at the source (https://github.com/r-prof/procmaps/blob/main/README.md?plain=1), it contains chroma class on the pre and span with attributes that Github doesn't know how to handle.

If downlit is able to do it, we could offer only auto linking on inline code as this will write markdown only:

This in Rmd

See `ggplot2::ggplot()`

becomes this in resulting md file

See [`ggplot2::ggplot()`](https://ggplot2.tidyverse.org/reference/ggplot.html)

Something like

output:
  github_document:
    code_linking: true

This way, we could have cleaner md file with half of downlit feature

@hadley did you have in mind specific behavior when closing r-lib/downlit#26 and r-lib/downlit#43 ?

[hadley]

@cderv nope, nothing specific that I can recall.