docs: change payer/payee to sender/recipient

[melissahenderson]

Instead of using payer/payee to represent the individual humans on each side of a payment, update the docs to use sender and recipient.

• Don't do a find and replace. Review each occurrence as the structure of a sentence may need to change.
• Sequence diagrams will also need to be updated.
• Consider using the tooltip component to define what a client is if it's possible the reader may interpret "client" as meaning "customer" or "recipient".
• Watch out for sentences that make it sound like something belongs to a sender/recipient rather than an ASE. For example, "The client requests a grant from the recipient's authorization server" isn't entirely accurate. The server belongs to the recipient's ASE.