Consistent documentation of an API using Julia and further programming languages

I am working on a C / Fortan interface (libtrixi) to the Julia-based flow solver Trixi.jl. We would like to provide a pleasing documentation for our users and have started with Documenter.jl as done for Trixi.jl. However, this does not allow to document the C / Fortran. We resorted to doxygen and now manually link to its output from a Documenter.jl site.

Does anyone know of any tool which would allow to generate a uniform documentation in one go? Any hints would be greatly appreciated.

I found a related, but not exhaustive, discussion here: