Add model spec for `proportional_hazards`

[hfrick]

use and rename {censored}'s cox_reg. make sure that Cox Model is mentioned appropriately in the documentation so it becomes googleable cc @EmilHvitfeldt

[EmilHvitfeldt]

Basically, Something useful (other than blog posts) should pop up if someone searches for "tidymodels cox model", or "parsnip cox model"

[github-actions]

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