Revised model documentation

[topepo]

This restructures the model documentation files so that

• Each engine has a separate man file (instead of being in the "Engine Details" subsection)
• Engines from parsnip-adjacent packages are automatically discovered and linked if they use the same conventions.
• The engines have more information than before (e.g., preprocessing requirements, references, etc).

Each engine-specific man file links to a Rmd file in man/rmd. The naming convention of the man pages is critical for finding them automatically.

Draft PR: only changes for linear_reg() and you'll need to run pak::pak("tidymodels/multilevelmod@doc-test") to see how other engines from other packages show up.

[DavisVaughan]

I like the idea! Mainly we'd just need some good documentation about how this works (would also be nice to mention that _ gets subbed for - when going from model name -> file name)

[hfrick]

Is it possible to get to an engine help page directly using its name? You might want to do that when you have already decided on the engine and don't want to click through from ?linear_reg. I tried ?details-linear-reg-glmnet but it didn't like the -. Could we use _ or does that break something?

[topepo]

Feedback from the tidymodels and tidyverse group meetings:

• Mention that the engines that are shown in the list are from loaded packages.
• We should list the names of other packages that are known to have engines for the model.
• We should keep an internal list of known packages with relevant engines and, if they are installed, show the engines anyway.
• Include the package name in the bulleted list.
• Add a "learn more at tidymodels.org" note on the pages (and in references)
• We can have a list of excluded packages that we know will never have the details files (e.g. survival, glmnet, etc). We should test the speed of the loading on a windows machine with a libPath on a network drive (or similar slow config).
• The details Rd files have names like details-linear-reg-glmnet. These should use underscores instead for easier autocomplete and simpler code.

[juliasilge]

What a set of changes 😅 but this is looking really great! 🙌

Other than the questions I have open above, I think the only other thing I want to check on in that we don't have these changes implemented for the survival models. That is as designed, correct?

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