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 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 be fixed and formatted with ruff before use

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...)

[mathause]

@TomNicholas do you think this has a chance to be considered. If not I happy to close. It's obviously only a fraction of the missing methods - can add them if this is considered.

One alternative is to generate the file once and then manually adapt the docstrings. That would be a bit less work than do everything by hand. (It's quite annoying to always write xr.map_over_datasets(lambda ds: ds.rename(...), dt) instead of dt.rename(...), etc. for many of the dataset manipulations.)