Create a contribution and developer guide for backports maintained by jaraco #123144
Description
In #123028 (comment)_, we identified a need to document better the methodology used by jaraco to maintain the backports of importlib.resources, importlib.metadata, and zipfile.Path, and maybe also other backports such as backports.tarfile, configparser, and singledispatch.
Also, let's make sure that the importlib.resources docs reference importlib_resources.