Adds open_dataarray to BackendEntrypoint.

[dncnmcdougall]

Description

This adds the capacity to implement open_dataarray in BackendEntrypoint. This is done in a backwards compatible way: if it is not implemented open_dataset is used instead (which is the current behaviour.) The documentation on open_dataset has been updated. The how-to-add-new-backend.md has been updated.

Checklist

• Closes Add open_dataarray to BackendEntrypoint? #10562
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst (Yes, they should be)

AI Disclosure

No AI was used in writing the code or documents in this pull request. AI may have been used for research.

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[dncnmcdougall]

I don't understand what is wrong with the document building.
There seems to be two errors

1. in the docstring of xarray/core/accessor_str.py:xarray.core.accessor_str.StringAccessor, (unexpected indent) and
2. an error in doc/user-guide/io.md. Maybe it is this:

 ------------------
ds = xr.tutorial.open_dataset("air_temperature_gradient")
cubes = ncdata.iris_xarray.cubes_from_xarray(ds)
print(cubes)
 ----- stderr -----
/home/docs/checkouts/readthedocs.org/user_builds/xray/checkouts/11537/.pixi/envs/doc/lib/python3.14/site-packages/ncdata/iris_xarray.py:53: SerializationWarning: saving variable Tair with floating point data as an integer dtype without any _FillValue to use for NaNs
ncdata = from_xarray(xrds, **xr_save_kwargs)
 ------------------

I do not know how to fix these:
For the first I did not touch that file, so cannot have altered it's docstring.
For the second all the other tests passed so I assume my change has not introduced this warning.

Are there other errors I missed? Is there something I can do to fix these, that is in scope for the changes for this PR?

Thank you.