Explain why N5Store is being removed

[dstansby]

In the docs, the deprecation notice for N5Store simply says

Deprecated since version 2.18.3: N5Store will be removed in Zarr 3.0.0.

This doesn't explain why this is happening, or if there are any alternatives, so isn't great for users (like me!) currently using N5Store. It would be good to add:

• Why the deprecation is happening
• What the alternatives are

[d-v-b]

we should put more content in there, e.g. directing users to n5py. That package also needs to be published in pypi iirc

[jhamman]

The in-code deprecation warnings do a better job of explaining where to go for more details: #2103