[Doc] Document node.optimize() from #230

[eddiebergman]

PR #230 introduce a convenient optimize() function that you can call directly on a pipeline. This feature is not fully complete as the EvaluationProtocol still needs to be fleshed out fully.

Once that is done, a rewrite of the optimization guide should probably be done to include these new features as this is the quickest way to go from pipeline definition to result.

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Blocked on implementing some EvaluationProtocol's for sklearn/openml.