feat: Cookbook on how to sign multiple inputs.

I am currently designing a stateless WASM wallet and using pure `rust-bitcoin` and `rust-bip39` only.
The cookbook was awesome to sign a transaction with a single input.
However, we also want to sign transactions with multiple inputs.
Maybe choosing which indexes of the `Vec<Txin>` to sighash/sign.

I might take this one if no one is up to (after I finish the implementation)