turn doc/definition into a specification

In CAAS6=7giX-z416wJOFebfViMMXoiTSCuOFw+iHXCCm9itFmO2A@mail.gmail.com, Marvin Humphrey (marvin@rectangular.com) commented:

The current definition would be a bit awkward to reference in an
official document or policy because it is not either frozen or
versioned.

Which makes me think we should do exactly that. Thanks to the SOURCE_DATE_EPOCH spec we have all the bits and pieces ready to make this a reality rather easy.