Adding content in Python API reference

[Jingru923]

As the outcome of #1439 we have now a frame for our Python API reference, but at the moment not a lot of content.

The API reference is generated by quartodoc, where it will grasp docstrings in the code and display them in the documentation.

To add more content in the API reference, one can simply add docstring that explaining the functionality of the attribute in Model() class.

[evetion]

Scope: I think all the classes/methods that we provide in our tutorials should have docstrings.