Use downlit

[mllg]

Not sure what would be required to make this work. Should also be used in the book.

https://github.com/r-lib/downlit

[be-marc]

downlit syntax highlighting and auto linking in multi line code boxes can be activated in the YAML section.

highlight_downlit: true

I will activate this in #102.

downlit + distill does not auto link inline code but this works

`r downlit::autolink("mlr3::Learner")`

We should stick to mlr3book::ref() because it allows us to have an alternative link text and the mlr3:: can be omitted.