Align datatree to_dict and from_dict to similar methods on Dataset and DataArray level

[etienneschalk]

• Closes DataTree: Align from_dict and to_dict behaviours to their Dataset equivalents #9074
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst
• Better examples in docstring for to_native_dict and from_native_dict

This Pull Request adds two new methods on the DataTree class: to_native_dict and from_native_dict.

~~The existing ones are renamed to to_paths_dict and from_paths_dict, as their working data structure is a dict of paths to xarray data structures. ~~

• DataTree.from_dict -> DataTree.from_paths_dict
• DataTree.to_dict -> DataTree.to_paths_dict

The goal is to convert back-and-forth DataTrees to Python dictionaries, with the final goal (left to the user) of dumping these dictionaries to JSON, and read back JSON to a DataTree. The motive is exposed in #9074

Open Points

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(1)

Since the DataTree.to_dict and DataTree.from_dict already exist on DataTree, but with a different behaviour, this change is a breaking change. I think it is needed for having a behaviour similar on the existing Dataset methods.

However, if the current usage of from_dict and to_dict is already so widespread that it cannot be renamed easily, then an option can be added to the from_dict and to_dict methods to switch between the current behaviour (convert a mapping of str paths to xarray objets to a DataTree) and the new (convert to a recursive dict with no left xarray structure, delegating lower-lever conversion of datasets to Dataset.to_dict and Dataset.from_dict

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