Add a section about rootlets to spinalcordtoolbox.com

Originally posted by @NathanMolinier in https://github.com/spine-generic/data-multi-subject/pull/158#issuecomment-2023835017:

> Also, I would like to add a small comment: I see that you are using the suffix _dseg because I imagine you are using discrete segmentations (label values may go higher than 1: not binary). However, ideally, the mapping between label values and the segmented structure should be referenced somewhere (on [spinalcordtoolbox.com](https://spinalcordtoolbox.com/user_section/tutorials/vertebral-labeling/labeling-conventions.html) like discs and vertebrae or directly within the dataset). So, is this mapping available somewhere ? Apart from that the filenames are good to go !

Start with the linked spinalcordtoolbox.com labeling-conventions page and determine where a rootlets section belongs. Document the mapping between discrete label values and segmented structures, with the section published on the website as the completion criterion.

Written by the indexing model from the issue text.