Explain how solvers can use the new helper

[acanidio-econ]

Solvers can now use the helper to create orders. The documentation should be updated to reflect this

https://github.com/cowprotocol/docs/blob/f4731189868c12d8220df2205e491134e9790b1f/docs/cow-amm/tutorials/cow-amm-for-solvers.md?plain=1#L24