Add `xarray` interface examples

See `xarray_from_image` and `image_from_xarray`

[https://github.com/InsightSoftwareConsortium/ITK/blob/master/Wrapping/Generators/Python/itk/support/extras.py#L627](https://github.com/InsightSoftwareConsortium/ITK/blob/master/Wrapping/Generators/Python/itk/support/extras.py#L627)

Start with xarray_from_image and image_from_xarray in Wrapping/Generators/Python/itk/support/extras.py at the linked location. Review the existing interface and the repository's example conventions, then add documented examples demonstrating both conversions. Done means the examples cover the requested xarray interfaces and build successfully with the project's documentation examples.

索引モデルが issue の本文から書いたものです。