Add public QuickPay React SDK documentation

[luo21660]

Summary

• Add a public QuickPay React SDK guide for Hosted Page links, Embedded Checkout, and Payment Element integrations
• Document checkout sessions as a payment source rendered through existing UI surfaces, not a fourth component type
• Cross-link the guide from the docs hub, quickstart, hosted checkout, API reference, FAQ, and integration guide

Notes

• Docs-only change under docs/
• Keeps merchant API credentials on the backend and treats browser callbacks as UX signals, not fulfillment proof
• Avoids internal PR, beta handoff, mock server, and quickpay-web implementation details

Validation

• Checked local Markdown relative links
• Scanned docs for internal PR/R2/quickpay-web/mock/local-path references
• Ran git diff --check