Include model.config property in the pyhf.pdf.Model API documentation

[alexander-held]

I would also suggest somehow pointing to _ModelConfig from the pyhf.pdf.Model page. The .config API is not listed there, and it is not particularly intuitive for a user to look at _ModelConfig to find out about that .config attribute, where it says

_ModelConfig should not be called directly. It should instead by accessed through the config attribute of Model

Given that _ModelConfig implies internal API, I wonder whether all of this page should just live within the Model API documentation page? This is where I would expect a user to look for it.

Originally posted by @alexander-held in #1972 (review)

[kratsg]

Given that _ModelConfig implies internal API, I wonder whether all of this page should just live within the Model API documentation page? This is where I would expect a user to look for it.

This part requires a larger rewrite of how we set up documentation. We currently set it up with one class per page (autoclass/autodoc). We'll probably change the way we do it in the future. But for now, this can be done by just linking to _ModelConfig.