Documentation and Tutorials

[storopoli]

This is the last PR to the initial release. Closes #2.

There is a nice quick start in the index.md along with all the docstring in the api.md.

The tutorials covered are "quick and dirty", we can improve more later.
They are:

• Linear Regression: linear_regression.md
• Logistic Regression: logistic_regression.md
• Poisson Regression: poisson_regression.md
• Negative Binomial Regression: negativebinomial_regression.md
• Robust Regression: robust_regression.md
• Hierarchical Models: hierarchical_models.md
• Custom Priors: custom_priors.md

I also added a favicon.ico and turing logo to the documenter pages.

[rikhuijzer]

The tutorials are very broken even though CI passes. There are two things we can do now:

1. Ensure that the docs build fails first and then fix the docs.
2. Switch to PlutoStaticHTML. I can set it up for you if you want. With PlutoStaticHTML, builds will fail if there is an error and we can provide readers with a notebook link which they can run locally.

[storopoli]

I all up for PlutoStaticHTML.jl. Can you convert the PR to use it?