Improve mach.json documentation & use the new markdown generator

[pineapplemachine]

The mach.json package currently uses the old markdown documentation generator; the documentation should be moved out of mach/json/readme.md and into e.g. mach/json/json.d and mach/json/package.d and the package directory should be removed from the list in utils/makereadme.d and added to the list in utils/makereadme2.d.

The documentation is also generally lacking and could particularly use a greater number of example unit tests, most of which could be almost straight copy-pasted from the package's existing tests.