docs: add payment data extraction example

[marlonbarreto-git]

Summary

Adds a new example for extracting structured payment data from unstructured text (bank statements, invoices, emails).

This example demonstrates several Instructor patterns applied to the payments domain:

• Pydantic models for payment transactions with custom field validators
• ISO 4217 currency code validation — ensures 3-letter uppercase codes
• PaymentMethod enum — constrains LLM output to valid payment types
• Cross-field model validation — verifies total matches sum of transactions
• Multi-currency handling — skips total validation when currencies differ
• Instructor retry logic — automatic re-prompting on validation failure

Motivation

Financial data extraction is a common use case for structured outputs. The existing extracting_receipts.md covers image-based receipt parsing, but there was no example for text-based payment data extraction with multi-currency support and cross-field validation patterns.

Changes

• docs/examples/payment_data_extraction.md — New example with full code
• mkdocs.yml — Added nav entry next to the receipts example