[FEATURE] document data requirements

[neuronflow]

We need something similar to this, like we have for the BraTS orchestrator. However, here we do not employ BET

## Data  Requirements
BraTS challenge algorithms require preprocessed brain scans.  
This preprocessing typically includes co-registration, brain extraction, and registration to a challenge-specific brain atlas (template).  
Please refer to each challenge’s documentation for details on which template to use.  

For convenience, we provide challenge-specific wrappers around our [preprocessing package](https://github.com/BrainLesion/preprocessing), part of the [BrainLesion Suite](https://github.com/BrainLesion).  
You can install the package with the preprocessing extra using:  


```bash
pip install brats[preprocessing]

The modular architecture of the BraTS Orchestrator facilitates employing your own preprocessing routines.
Alternatively, the preprocessing package can be used independently to design custom preprocessing pipelines tailored to your specific needs.

[neuronflow]

this is necessary as we see in #21