Guide to converting code from zarr-python v2 to zarr-python v3

[dstansby]

As part of the v3 release, there should be a complete and clear guide on how to update code written for v2 to v3. This should include (or link to) a list of all API changes, that I don't think exists at the moment?

[jhamman]

Closing this as a duplicate of #1798

[jhamman]

Sorry, #1798 has more in it than just the guide. We can reopen this to track the migration guide.

[jhamman]

closed by #2596