Add model analysis .Rmd template

[juliasilge]

This PR adds an .Rmd template to the tidymodels metapackage to demo an example model analysis. The use case here is that people choose it from the RStudio menu when they are starting a modeling analysis and it helps folks get started with their own problem.

As of now, consider this a sort of straw man for us to discuss and edit. Some questions I have:

• I included an example with no tuning. Thoughts?
• Same for a recipe; I chose to keep it simpler and link out to learn more
• What do we think about the level of narrative? Too much? Too little?
• Anything that you would want to include or take out?

[juliasilge]

I changed the .Rmd template to use the new extract_workflow() function, and bumped the required workflows version to devel. 👍

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