Unit tests and examples: XGBoost

[cosmicBboy]

• extend the integration test suite to include a xgboost-based app: https://github.com/unionai-oss/unionml/tree/main/tests/integration
• update the documentation index.md to include a new xgboost tab: https://github.com/unionai-oss/unionml/blob/main/docs/source/index.md

[Abdullahi-Ahmed]

#self-assign

[Abdullahi-Ahmed]

#self-assign