[codex] Improve Paysera adoption docs

[nahremenkova1]

Summary

• Add Node/NestJS setup notes and a checkout-first quick start for the first order flow.
• Document per-environment webhook registration, raw-body NestJS handling, and async module configuration.
• Add npm discovery keywords for checkout, webhooks, and refunds.

Validation

• package.json parsed with node
• npm pack --dry-run --json
• README.md Markdown fence balance check