docs: Add support for class methods in API reference

[AkeemMcLennon]

Context

This pull request adds support for class methods in the api documentation by replacing calls to inspect.isfunction() with the more inclusive inspect.isroutine(). In particular, the Image, Prediction and Signature primitives have several relevant functions that were not being captured by the previous method.

It also excludes functions defined by external modules (e.g. Pydantic), but it seems there is some other mechanism that also filters them out somewhere, since they do not appear in the existing documentation despite being present in the generated markdown.

Before

After

[chenmoneygithub]

Thanks for the PR, this is pretty good!