fix: correct Solo checkout example in affiliate keys docs

[nicosommi]

Summary

• Fixes the Solo Reader Checkout example in affiliate keys docs: nested checkout / decimal amount did not match CreateReaderCheckoutRequest and omitted required affiliate.foreign_transaction_id, which caused 400s for copy-paste integrators.
• Aligns the example with OpenAPI (total_amount, root description, affiliate app_id / key / foreign_transaction_id) and clarifies this payload is for Cloud API POST .../readers/{reader_id}/checkout, not mobile SDKs.
• Replaces the absolute developer.sumup.com link with root-absolute /api/readers/create-checkout/.

Caveat: OpenAPI marks affiliate nullable at the request root; product docs treat Affiliate Keys as mandatory for card-present. The example includes affiliate with the subfields required when that object is sent.

Test plan

• npm run check
• npm run lint:markdown
• npm run linkcheck
• npm run build
• Preview /tools/authorization/affiliate-keys/ and confirm JSON + link render

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	developer-sumup-com	40da4ca	Commit Preview URL Branch Preview URL	Oct 09 2026, 01:36 PM