Add CMIP6 datatree sample

[andersy005]

• Towards Example datatree for use in tutorial documentation xarray-contrib/datatree#100
• Notebook used to generate this dataset: https://github.com/andersy005/brouillons-quotidien/blob/main/datatree/cmip6-sample.ipynb

Cc @TomNicholas

[keewis]

just wondering: should we put datatree examples into their own subdirectory? At the moment, anything in the repository root is picked up by xr.tutorial.open_dataset.

[andersy005]

👍 for a separate folder

[kmuehlbauer]

@andersy005 For some reason your change is already committed in master, but this PR still exists.

[andersy005]

@andersy005 For some reason your change is already committed in master, but this PR still exists.

interesting... not sure how they got there. perhaps someone else added them. happy to close this one to reduce noise