add engine docs for bonsai wrappers

[simonpcouch]

The lightgbm docs are pretty thorough, but the partykit docs are mostly barebones / copy+pasted from similar party docs—wanted to give @topepo a chance to drop any notes from implementing those wrappers.

Also potentially fixes a previously documented bug in translate.model_spec.

library(parsnip)
library(bonsai)
library(censored)
#> Loading required package: survival

rand_forest() %>% 
  set_engine("partykit") %>% 
  set_mode("classification") %>% 
  translate()
#> Error in names(args) <- merged$original: 'names' attribute [4] must be the same length as the vector [3]

^{Created on 2022-05-10 by the reprex package (v2.0.1)}

This seems to arise from both bonsai and censored supplying implementations here, with differing metadata (in this case, has_submodel). Dropping duplicated orders makes the length of the results and joined data giving the names equal.

library(parsnip)
library(bonsai)
library(censored)
#> Loading required package: survival

rand_forest() %>% 
  set_engine("partykit") %>% 
  set_mode("classification") %>% 
  translate()
#> Random Forest Model Specification (classification)
#> 
#> Computational engine: partykit 
#> 
#> Model fit template:
#> parsnip::cforest_train(formula = missing_arg(), data = missing_arg(), 
#>     weights = missing_arg())

^{Created on 2022-05-10 by the reprex package (v2.0.1)}

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