consider renaming ?details_model_type_engine docs to ?model_type_engine

[simonpcouch]

After a couple weeks away from parsnip, I was trying to remember the convention for finding engine-specific help-files. I eventually remembered it was ?details_model_type_engine, e.g. ?details_linear_reg_glmnet, but this made me wonder if just using ?model_type_engine, e.g. ?linear_reg_glmnet, would make these more discoverable, as folks are probably thinking to type ?linear_reg to search for that documentation in the first place and would get autocomplete then.

For back-compatibility, this would probably just be an alias rather than a proper rename.