Add a Two-Step BART Adjustment Vignette

[andrewherren]

Add a multilingual vignette on a "two-step" BART model with an additive linear regression term which is used as a covariate, in the style of this PR, where we model:

$$y = X \beta + f(X, X \beta) + \epsilon$$

with $f(X)$ as a stochastic tree model, $\beta$ as a vector of linear regression coefficients, and $\epsilon \sim N(0,\sigma^2)$

We fit the model in two stages:

1. $\beta$ is estimated by $y = X\beta + \nu$ linear regression
2. $f$ is sampled via $y - X \beta = f(X, X\beta) + \epsilon$, where $\epsilon \sim N(0,\sigma^2)$