My drop in the bikeshed bucket:
I think one of the most important things about documentation tooling is to make it trivial for people to fix documentation issues they come across. Requiring users to learn a new format is non-trivial.
For this reason alone my opinion is that Markdown is the way to go because everyone knows it and almost everyone uses it for documentation. It’s not about the number of features and ways to generate different highlights if those don’t meaningfully improve the quality of the documentation.
Most of the docs issues will always be about how to structure them, how the text in them is phrased etc. and that should outweigh everything else.
I’ve started writing some Nix language documentation in Markdown (nix-1p) and will add more over time as I integrate these into the Nixery documentation.