Document how to migrate away from init_group in v3

[dstansby]

I'm trying to upgrade to v3, and getting
E ImportError: cannot import name 'init_group' from 'zarr.storage'

This isn't mentioned on the migration guide - would be great to add something about why init_group was removed, and what the alternative is.

[jhamman]

One might ask why init_group was in zarr.storage but alas, I suppose it just was 🤷 .

I suspect we would have heard from folks if they were using this but if needed, we could reimplement it in the top level API. Or, as you say, we could document its removal in the migration guide.