Unit tests and examples: Huggingface

[cosmicBboy]

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