Low-level signing documentation

[mDuo13]

The Serialization docs describe to a good level of detail how to create the binary format of transactions. They don't cover in sufficient detail how to go from there to a signed transaction that can be submitted to the network.

Ideally, the documentation would include:

• Steps to calculate the signature from the JSON (add the signing public key, calculating a with the correct prefix, etc.)
• Differences for multi-signing (different prefix, SigningPubKey empty, must append the accountID to the hash—thanks, josepajay on XRPChat for the note on the last one)
• Links / mentions of best practices for supported signing algorithms for example using deterministic nonces for secp256k1 signatures per RFC6979)
• Sample code of the whole process (perhaps importing / depending on the serialization.py sample code)

[mDuo13]

One note that would be useful in this doc: secp256k1 signatures are DER-encoded (because that's how Bitcoin did it) but Ed25519 signatures are not (with how the Ed25519 scheme defines its signatures, DER-encoding is not necessary or useful)

[mDuo13]

Incomplete, slightly dated draft on this topic: https://github.com/XRPLF/xrpl-dev-portal/blob/master/content/concepts/payment-system-basics/transaction-basics/understanding-signatures-draft.md