Add spinal_cord and spinal_canal folders

[valosekj]

During the 2025-01-08 DCM norm meeting, we decided on the reorganization of this repository:

• move the CSV files with SC morphometrics from the root directory into a new directory, spinal_cord
• once we have morphometrics also for the spinal canal, save them a spinal_canal directory

As this repo is downloaded and used by SCT, we will probably need to reflect these changes in the SCT code.

Steps:

• move the CSV files with SC morphometrics to the spinal_cord directory - done in #16
• publish a new release
• update release link in SCT's download.py : https://github.com/spinalcordtoolbox/spinalcordtoolbox/blob/e293dca0512d7972fd1598a7c454aceeb0f40f74/spinalcordtoolbox/download.py#L159
• update path in sct_compute_compression.py: https://github.com/spinalcordtoolbox/spinalcordtoolbox/blob/e293dca0512d7972fd1598a7c454aceeb0f40f74/spinalcordtoolbox/scripts/sct_compute_compression.py#L560