Document directories where different MPI builds should go into

[Neves-P]

[...W]e should just explain in the documentation really well what type of MPI build host sites should put in what directory (i.e. that for the ROCm case, it should be a ROCm-enabled LLVM compiling a ROCm-enabled MPI and putting it into the system-rocm-rocm-<rocmversion> dir).

Originally posted by @casparvl in EESSI/software-layer-scripts#231 (review)

[ocaisa]

Indeed, I think in the end the documentation for the injection of MPI is going to be more of a best practice guide: you can do many things, but there is what I would do...