Duplicate documentation of stores

[dstansby]

Searching e.g., memorystore in the v3 docs shows that stores seem to be documented twice in the API docs: https://zarr.readthedocs.io/en/latest/search.html?q=memorystore

I think the fix here is to clean up the store namespacing, so all the public API is in zarr.store instead of the sub-modules.