This PR improves the model card by:
- Linking to the research paper: [Primus: Enforcing Attention Usage for 3D Medical Image Segmentation](https://huggingface.co/papers/2503.01835).
- Updating the `pipeline_tag` to `image-segmentation` to accurately reflect the model's purpose.
- Providing a link to the official GitHub repository and documentation.
- Organizing the model variants and SSL methods for better readability.