Update UDF signature docs for apply_datacube also supporting xarray.DataArray

Triggered from discussing #533 with @VictorVerhaert :
the docs on UDF signatures (https://open-eo.github.io/openeo-python-client/udf.html#module-openeo.udf.udf_signatures) should be updated to newly added xarray.DataArray support (#310)

I wanted do it naively by introducing Union[XarrayDataCube, xarray.DataArray] in
https://github.com/Open-EO/openeo-python-client/blob/7ce92925dd9544ff116b14c0339de203f854ec27/openeo/udf/udf_signatures.py#L30

but then realized that that is actually more confusing, as the user is not expected to use Union there.

The user should pick one of

def apply_datacube(cube: XarrayDataCube, context: dict) -> XarrayDataCube:
#or 
def apply_datacube(cube: xarray.DataArray, context: dict) -> xarray.DataArray:

but defining it like that in udf_signatures.py is not going to work (first form will be overwritten by second).

We probably have to port udf_signatures.py from a python file to a RestructuredText file in the docs

Start with openeo/udf/udf_signatures.py and the UDF signatures documentation at the linked page. Update the documentation to show separate apply_datacube signatures for XarrayDataCube and xarray.DataArray without suggesting that users should write a Union; the result should accurately describe the newly supported DataArray form.