Add column to indicate how easy it is to download a dataset

[jcohenadad]

Some datasets are easier to find than others. Eg: OpenNeuro is a single click, while others require paperwork, request, slow processing time, etc.

I suggest we add a column, right after "Dataset Name", with an emoji that indicates how easy it is to get the dataset. Eg:

To add on top of the README:

### Access legend
- 🟢 Easy: Immediate download (no approval required)
- 🟡 Moderate: Registration or DUA required (quick access)
- 🟠 Controlled: Application review needed
- 🔴 Restricted: Difficult or long access process
- ⚫ Unknown: Access conditions unclear

It would look like this:

Dataset Name		Region	# Subjects	Contrast	Usecase
Duke-CSpineSeg	🟡	Cervical	1255	T2w	Structural / Seg
Spine Generic	🟢	Multi-center		T1w, T2w, T2*w, DWI	Multi-vendor
Lumbosacral	🟢	Lumbosacral	14	T2TSE, DESS, CISS	Nerve Root Res

[jankirschke]

Dataset Name		Region	# Subjects	Contrast	Usecase
NAKO	🟠	Whole-spine	30k BL, 20k FU	T2w sag	German population study
SHIP	🟠	Whole-spine	ca. 5k, different cohorts	T1w, T2w sag	North-German population study

[jcohenadad]

@jankirschke in reference to #7 (comment), these would be great additions indeed-- thank you!