RSpec does something along that vein with the documentation on https://relishapp.com/rspec. E.g., here's the documentation for predicate matchers: https://relishapp.com/rspec/rspec-expectations/v/3-1/docs/bu..., and here's the executable Cucumber file from which that page is generated: https://github.com/rspec/rspec-expectations/blob/master/feat....

Maybe an unnecessarily meta example, given that it's the executable documentation/test suite for a testing framework itself. But it's wicked cool!