WIP: Document current approach to permissions

[maxrjones]

An attempt to document the current approach to permissions in the Zarr python library.

TODO:

• [ ] Add unit tests and/or doctests in docstrings
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions
• [ ] New/modified features documented in docs/user-guide/*.rst
• [ ] Changes documented as a new file in changes/
• [ ] GitHub Actions have all passed
• [ ] Test coverage is 100% (Codecov passes)