[Cashtab] About NFToa Protocol ID

Summary:
Add documentation file for the NFToa protocol ID at doc/standards/nftoa.md

The latest update of this documentation triggered the need to revise the functions in D18785

Test Plan: Checked that the new nftoa.md file renders correctly in Markdown. No code changes are involved.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien, bytesofman

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien, bytesofman

Subscribers: nftoa, bytesofman, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18757