Improve documentation on the datalist format #8525
Description
Is your feature request related to a problem? Please describe.
Currently, the documentation is quite opaque about the (Decathlon) Datalist format, which is required for custom datasets for e.g. the Auto3dSeg AutoRunner. Currently, the only way for users to become sufficiently familiar with this format is by downloading one of the Decathlon datasets and studying its .json files' structure.
Describe the solution you'd like
An improved documentation on the format, e.g. under the Decathlon Datalist loader method: https://docs.monai.io/en/stable/data.html#decathlon-datalist
(note that the documentation seems incomplete there anyway)
This is something I could take up to implement.
Describe alternatives you've considered
There is some description of the format in the run_with_minimal_input tutorial, but this is incomplete, as it does not mention the train and test lists, and any other metadata in the file format.
Additional context
See discussion: #8521