generate datatree methods

[mathause]

• Closes DataTree: missing methods #10015
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

Add a script that generates a mixin so Dataset methods are available on DataTree. Uses inspect.signature to re-generate the call signature and a decorator so we can still use *args, **kwargs, and so we don't need to populate the method body, making the generation relatively trivial (although maybe not trivial to understand).

This is much clunkier than generate_ops or generate_aggregations. However, we cannot profit from common signatures. Thus

• the docstring is not adapted
• the examples are not adapted
• the generated file needs to fixed and formatted with ruff

However, it's a fraction of the work to do this properly. I am really not sure if this is a good idea - feel free to tell me it's not!

[mathause]

The alternative is to inject everything (as in https://github.com/xarray-contrib/datatree/blob/5f3956ffe80e686dd3df54ee8cef9ff56c158e76/datatree/ops.py#L223). (Or to write all methods out, or create mixin classes that work for all data types...)