Merge `packaging_ventral_rootlets` and `packaging_lumbar_rootlets` folders

Currently, the repo contains the following two folders:

- [packaging_ventral_rootlets](https://github.com/ivadomed/model-spinal-rootlets/blob/main/packaging_ventral_rootlets)
- [packaging_lumbar_rootlets](https://github.com/ivadomed/model-spinal-rootlets/tree/main/packaging_lumbar_rootlets)

These folders contain instructions on how to use beta models (not part of SCT yet)

Their content is pretty similar; for example, `run_inference_single_subject.py` is almost identical for both folders. I believe I created two folders because I shared different models (cervical, lumbar) with different users. But it would be better to merge the folders into something like `packaging` to reduce duplicities.

TODO:
- [x] wait for merge of https://github.com/ivadomed/model-spinal-rootlets/pull/53