document elastic net regularization via mixture

[simonpcouch]

Closes #570. In my experience, "interpolate" is the verbiage here. :)

Will need someone else to generate documentation for me—spent a while trying to debug, and eventually came across a set of errors in knit_engine_docs:

Failed with error:  'package 'parsnip' required by 'censored' could not be found'
Failed with error:  'package 'parsnip' required by 'discrim' could not be found'
# etc etc

that's not worth me troubleshooting if others are able to follow inst/README-DOCS.md just fine.🛫

[topepo]

Try using parsnip:::install_engine_packages().

Where should we document this?

[simonpcouch]

Aha—I think load_all got me here. Was able to knit engine docs after local install + restart. :)

Where should we document this?

Follow-up from conversation with Max: This is indeed documented here!

[EmilHvitfeldt]

LGTM!

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