Add tidymodels terminology overview vignette

@dshemetov and I were confused running through some vignettes without learning some tidymodels stuff that didn't seem trivial to google. It would help to have a vignette either pointing to pages that explain tidymodels essentials, or explaining tidymodels essentials. E.g.:
* Note if you just want to predict, just need to fit and predict on epi workflow.
* To view preprocessed training data, do this: {code}
* To view preprocessed testing data, do this: {code}
* Flowchart of steps, recipes, prepped recipes, etc.
* Migrate the role explanation from one of the other vignettes here?