Improve API documentation

[jobrachem]

Our API documentation currently is very limited. I think it would be a good investment of our time to improve it. Specifically, that means:

1. Describe not only what a class/function does, but also put it into context. For example, the lsl.GooseModel is currently described with the sentence "A ModelInterface for a Liesel Model". This sentence adds almost no information on top of the class name. How do you use it? What job does it do?
2. Classes and methods should be equipped with examples, showcasing common usage.
3. Where possible, examples should be written as doctests, because that makes sure that examples stay up to date.
4. We should make frequent use of the "See Also" section.
5. All parameters should be documented.
6. All returned values should be documented.

This issue tracks our progress with improving the documentation.

I selected what I think are the most important starting points for improved documentation.

Liesel issues

• Improve API documentation of lsl.GraphBuilder #114
• Improve API documentation of lsl.Model #115
• Improve API documentation of Nodes #116
• Improve API documentation of liesel.model.viz #117

Goose issues

• Improve API documentation of gs.LieselInterface #113
• Improve API documentation of gs.EngineBuilder #118
• Improve API documentation of gs.NUTSKernel #119
• Improve API documentation of gs.IWLSKernel #120
• Improve API documentation of gs.models #121
• Improve API documentation of gs.RWKernel #122
• Improve API documentation of gs.GibbsKernel #123