This is a great idea; I think improving the Dune docs is probably one of the most impactful projects that an Outreachy intern could take up, and all in all a great service to the community (as well as a good way to learn about how the OCaml toolchain works!).
The current Dune docs are mostly an enumeration of all possible stanzas with some added commentary around; it is closer to a reference manual than to a user guide. It is sorely lacking “tutorial” content for beginners. In this regard, I always remember fondly the SCons user guide SCons 4.6.0, a good source of inspiration for a project like this in my opinion.
Cheers,
Nicolas