Documentation & Examples

Parent Epic: #155

# Task: Documentation & Examples

## Description
Update all docstrings, create usage examples, and write migration guide for users upgrading to the new parameter-enabled version.

## Acceptance Criteria
- [ ] All parameter docstrings complete
- [ ] Usage examples for each parameter
- [ ] Migration guide written
- [ ] API reference updated
- [ ] README examples updated

## Technical Details
- Update docstrings in htfa/core/htfa.py
- Create examples/ directory with notebooks
- Write migration guide in docs/
- Show backend selection examples
- Document performance implications

## Dependencies
- [ ] Task 162 must be complete (all features tested)

## Effort Estimate
- Size: S
- Hours: 4-6
- Parallel: false

## Definition of Done
- [ ] Docstrings updated
- [ ] Examples created
- [ ] Migration guide written
- [ ] README updated
- [ ] Documentation reviewed