Add documentation about best practices for library documentation

[MarcCoquand]

We have https://ocamlverse.github.io/content/documentation_guidelines.html that explains how to add documentation to Ocaml. However it's not immediately obvious how documentation in Ocaml translates to documentation in Reasonml. Should ocamldoc be used? Should documentation go in .rei or .re? In Ocaml comments also go under the function which is a bit different from other langugeag.s It would be nice if this was explained in the documentation.

[petevdp]

+1
The doc comment syntax should be included on-site somewhere as well.

I'm very new to ReasonML and its ecosystem, but I could write a PR for this if I got some general direction on it.