add examples for dataset_conversion for TCIA DICOM SEG/RTSTRUCT datasets

[kirbyju]

Hi there!

In The Cancer Imaging Archive we host a large number of DICOM datasets that have images (CT, MR, PT, etc) with corresponding tumor and organ segmentations (RTSTRUCT, SEG). Would it be possible to create some examples that help people get from those DICOM datasets to something that nnUNet expects as input? I'd be happy to hop on a t-con to discuss some details if there is interest.

Best,
Justin