[FL] Vertical FL

### Book

fundamentals

### Pocket Reference Title

Vertical FL (`vertical/readme.md`)

### Proposed Content

TBA

### Rationale

TBA

### Content Types

- [x] Theoretical foundations
- [x] Mathematical formulations
- [x] Code examples
- [x] Diagrams/visualizations
- [x] Practical applications
- [x] Common pitfalls/challenges

### Additional Resources

TBA