document the duck array integration status

[keewis]

With the initial (experimental) release of pint-xarray getting closer (see xarray-contrib/pint-xarray#25), I figured it might make sense to document the parts of the API of xarray which right now won't work with duck arrays, and to start maintaining a list of extensions like pint-xarray.

The remaining issues are taken from #3594, but I'm sure I missed something (e.g. plotting).

• Passes isort . && black . && mypy . && flake8
• User visible changes (including notable bug fixes) are documented in whats-new.rst

[keewis]

I'm not sure "plotting" should be included in that list since converting to numpy is intentional.

Other than that, this should be ready for review.

[keewis]

would it make sense to point to the explicitly tested duck arrays here (pint, sparse)? We could also add a user-maintained list of duck array libraries, just like the current "related projects" list.

I did think about adding usage examples, but maybe it's better to leave that to the extension packages?

[dcherian]

I think a list would be nice.

[max-sixty]

Great idea, thanks @keewis

[keewis]

I just noticed that we have to wrap chunk calls with a dequantify / quantify cycle if we want to have it work correctly, but maybe that's something that belongs into the pint-xarray docs?

[keewis]

if there are no further comments, should we merge this? I'm almost certain the list of missing features is not complete, but I would open other PRs once new items are found.

[dcherian]

Thanks!