chore: Add instruction on how to build a ComposableCoW signature

[fedgiac]

Background

The ComposableCoW docs implicitly assume that all orders are generated from getTradeableOrderWithSignature and there's no instruction on how to build a valid signature for an order.

Details

This is particularly relevant for the CoW AMM: we expect solvers to create custom orders through a contract that relies on a custom verify implementation. Right now, the best I can do is linking to the code of getTradeableOrderWithSignature.

Acceptance criteria

• I can share a link with details on how to create a valid signature for ComposableCoW.