[v3] Documentation: How to extend Zarr

[jhamman]

Zarr v3 will have a number of formal extension points and new options. This documentation ticket should explain how developers may choose to extend Zarr:

Some sample topics:

• how to write a custom store
• how to write a custom codec pipeline
• how to subclass array / group (e.g., to have typed attributes, typed members)
• how to get good performance
• future extension points
  • storage transformers (e.g. #1718)
  • chunk grids (e.g. #1752)