apis.json for versions of MPI standard should be public

[hppritcha]

MPI implementors are making increased use of the mpipython code to generate compilable fortran interfaces and c function prototypes. This code relies on an apis.json file generated from the MPI standard latex source. It would be helpful if the apis.json file for each released version of the standard (perhaps starting with 4.0) were made publically available.

[hppritcha]

@jsquyres

[jsquyres]

Additionally, a formal schema for the JSON should be published as well.

Right now, it's just generated JSON from a single python code (https://github.com/mpi-forum/pympistandard/). That's probably good enough, honestly -- but let's at least document the schema of the JSON that it produces so that other downstream software (e.g., MPI implementations and MPI bindings projects) can reliably use the published JSON.