Dynamically include different modes in engine-specific documentation

[hfrick]

Currently the documentation is designed to dynamically show which engines are available based on the loaded packages. However, the modes covered by these engines are not dealt with dynamically.

Example:
For decision_tree(), parsnip contains the rpart engine for modes classification and regression. censored includes an rpart engine for the censored regression mode but can't include documentation for it easily.

[juliasilge]

This was addressed in #631, right, @hfrick?

[hfrick]

yes, it was 👍

[github-actions]

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