Add ONNX example: PLA Sigmoid (replace lookup with piecewise linear)

[changshenhan]

This adds an ONNX example under examples/onnx/pla_sigmoid that replaces Sigmoid lookup with a piecewise linear approximation (PLA). The circuit uses only mul/add/compare (copy and arithmetic constraints), no lookup.

• Keeps accuracy ≥ 99.5% vs true sigmoid.
• Allows lighter ezkl settings (num_inner_cols=1, minimal lookup_range).
• Useful as a reference for reducing proof cost when handling nonlinear ops.

Author: @changshenhan

[changshenhan]

Hi @jasonmorton and @zkonduit-team,

I've submitted this PR to implement the Piecewise Linear Approximation (PLA) for Sigmoid, which significantly optimizes the proof generation time.

It's been a few days and the CI workflows are still awaiting approval. Could you please help to run the workflows so I can ensure everything passes on your side?

I've already verified the logic locally with a precision of 99.5%. Thanks for your time!