DOC: Add new backend page does not mention DataTree

[ianhi]

What is your issue?

The adding a new backend docs page: https://docs.xarray.dev/en/stable/internals/how-to-add-new-backend.html

does not mention DataTree despire Backends being required to implement:

https://docs.xarray.dev/en/stable/generated/xarray.backends.BackendEntrypoint.html#xarray.backends.BackendEntrypoint

open_datatree method: it shall implement reading from file, variables decoding and it returns an instance of DataTree. It shall take in input at least filename_or_obj argument. The implementation of this method is not mandatory. For more details see .