Write usage documentation

## Summary
Write comprehensive documentation for using ccgram.

## Success Criteria
- [ ] Installation instructions (pip install, from source)
- [ ] Quick start guide with examples
- [ ] API reference documented
- [ ] CLI usage documented (if applicable)
- [ ] Configuration options explained
- [ ] Troubleshooting section
- [ ] Contribution guidelines

## Documentation Sections
1. **README.md**: Update with installation and quick start
2. **docs/getting-started.md**: Detailed setup guide
3. **docs/api.md**: API reference
4. **docs/configuration.md**: Config options
5. **CONTRIBUTING.md**: How to contribute

## Examples to Include
\`\`\`python
from ccgram import generate_commit

# Simple usage
message = generate_commit(diff="added login button")
# Output: "feat(ui): add login button"

# With options
message = generate_commit(
diff="fixed null pointer in auth",
type_hint="fix",
scope_hint="auth"
)
\`\`\`

## Quality Bar
- A new user can install and run in <5 minutes
- All public APIs documented
- Examples are copy-pasteable