improve docs and errors re: model formulas

[simonpcouch]

Closes #770. Closes #1014.

This PR adds documentation and improves errors surrounding the "model formula." While users would have to know about the add_model(formula) argument already to find its docs, my hope here is to make a more general and findable resource.

library(tidymodels)
  
gam_wflow <- workflow() %>%
  add_formula(mpg ~ .) %>%
  add_model(gen_additive_mod("regression"))

gam_fit <- gam_wflow %>% fit(mtcars)
#> Error:
#> ! When working with generalized additive models, please supply the model
#>   specification to `workflows::add_model()` along with a `formula` argument.
#> ℹ See `?parsnip::model_formula()` to learn more.

gam_res <- gam_wflow %>% fit_resamples(bootstraps(mtcars))
#> → A | error:   ! When working with generalized additive models, please supply the model
#>                  specification to `workflows::add_model()` along with a `formula` argument.
#>                ℹ See `?parsnip::model_formula()` to learn more.
#> There were issues with some computations   A: x25
#> 
#> Warning: All models failed. Run `show_notes(.Last.tune.result)` for more
#> information.

^{Created on 2023-11-02 with reprex v2.0.2}

I'd also recommend we link to this page in the following (dev) recipes error:

library(recipes)
recipe(mpg ~ cyl + hp / 1, mtcars)
#> Error in `inline_check()`:
#> ✖ No in-line functions should be used here.
#> ℹ The following function was found:
#> • `/`
#> ℹ Use steps to do transformations instead.

...as well as in workflows' add_model() help-file. We also ought to link ?model_formula in engine docs, but this PR is getting pretty large already; I'll leave that for another PR.

Ask: do we have a word we've already been using for "preprocessor formula"?

[hfrick]

This will be good to have 🪆

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