Duck array documentation improvements

[TomNicholas]

Draft improvements to the user guide page on using duck arrays.

Intended as part of the scipy tutorial effort, though I wasn't sure whether to concentrate on content in the main xarray docs or the tutorial repo.

(I wrote this on a train without enough internet to update my conda environment so I will come back and fix anything that doesn't run.)

• Part of SciPy 2023 tracking issue xarray-contrib/xarray-tutorial#170
• User visible changes (including notable bug fixes) are documented in whats-new.rst

cc @dcherian and @keewis

[dcherian]

Thanks @TomNicholas

[dcherian]

LGTM! Thanks Tom, this is a great addition. Just some minor comments.