docs: Missing API reference for bentoml.Model

[eliorc]

Describe the bug

As a part of the integration of BentoML, we need to automate the process of importing and exporting BentoModels.
Examining the documentation, specifically the API Reference section, there is no mentioning of the ability to do so through the Python SDK.
Examining the code in this repo, we found bentoml.Model.export and bentoml.Model.import_from but they are not documented anywhere in the API reference.

To reproduce

Irrelevant

Expected behavior

Anything present int he SDK should be found in the API Reference part of the documentation

Environment

Irrelevant