Document how to release

[hammer]

@tomwhite wrote up some great instructions at https://github.com/pystatgen/sgkit/issues/608#issuecomment-891117795. We should probably put these instructions somewhere more permanent, though I'm not quite sure where. Perhaps as a discussion at https://github.com/orgs/pystatgen/teams/committers?

[tomwhite]

Maybe as a section on https://pystatgen.github.io/sgkit/latest/contributing.html?