Add tidymodels terminology overview vignette

[brookslogan]

@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?

[brookslogan]

• how to get training data matrix