Add more example notebooks

[Erik Jenner]

We should at least cover:

• Showing how to use WaNet correctly without shooting yourself in the foot
• Demonstrating adversarial examples
• Demonstrating a detector that uses some untrusted training data

Less crucial, but ideally we'd also have some actually interesting examples, e.g. non-trivial ablation evals or running a bunch of task/detector combinations at once.