Documentation improvements

[Gattocrucco]

Things to do to improve the documentation:

• examples and tutorials
  • something that shows how to run the MCMC directly, without the BART interface
  • something that shows how to deal with predictors
    • how to choose the cutpoint grid
    • dummies
    • collinearity
• split the internal reference in multiple pages
  • right now there are a few html pages with many functions listed in each, this is hard to navigate
  • alternatively, long pages would be fine if there was an index or a sidebar
• use a more modern html template
  • copy what jax does
• write a math reference
  • There is no complete reference for the math in the BART MCMC. There are some scattered pieces here and there. Recently I fixed a problem (b3d8a2f) in the MCMC that originated in a formula I copied from the appendix of the bartMachine paper (Kapelner & Bleich 2016, JSS), which was subtly wrong. So a precise reference may be quite useful to the BART community.