[v3] Documentation: describe how Zarr-Python approaches consistency and synchronization

[jhamman]

We need to write some documentation that clearly states how Zarr-Python v3 approaches consistency and synchronization.

Topics to cover:

• Concurrent readers and writers
• Synchronization (async, threads, processes, distributed)
• What is cached and what is not
• How to force Zarr to refresh its view of the underlying store

Writing this will, I believe, help zarr-python be less defensive and reduce unnecessary traffic to the store. It will also help developers understand constraints when working with zarr in complex environments.