allow documentation callbacks

[t-bltg]

Provide a way to give a documentation callback, when parsed c documentation is absent, implementation is tbd:

options = load_options(joinpath(@__DIR__, "generator.toml"))

options["general"]["callback_documentation"] = node -> String["This is the docstring generated for $(node.id)"]

For JuliaParallel/MPI.jl#652.

[t-bltg]

@Gnimuc, I've added specific tests for the configuration we use in MPI.jl.

Those also cover the previous PR: #385.

CI script was also refreshed and simplified.

[Gnimuc]

I will do a squash merge if there are no additional changes.

[t-bltg]

I'm done, pending CI does not fail.
Also, a version bump to 0.16.3 would be appreciated for our Project.toml (sorry for having to bump twice recently).

[t-bltg]

Thanks all for the valuable comments ;)