[codex] Improve Klix adoption docs

[nahremenkova1]

Summary

• Add a checkout-first quick start that explains redirect plus callback/webhook confirmation.
• Expand webhook setup with a repeatable code-created webhook and NestJS callback/webhook verification routes.
• Add npm discovery metadata for checkout, webhooks, refunds, homepage, and bugs URL.

Validation

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