Add Xendit API docs (Node.js/TypeScript)

[RuniVN]

Adds community docs for the Xendit Node.js SDK v7.

Covers all 8 modules:

• Invoice — payment links with multi-method support
• PaymentRequest — unified API for e-wallet, QR, VA, cards
• PaymentMethod — reusable/tokenized payment methods
• Refund — create, list, cancel
• Transaction — query transaction history
• Balance — CASH/HOLDING/TAX balance
• Customer — create and manage customers
• Payout — disbursement to banks/e-wallets with channel codes for ID, PH, MY, VN, TH

Also includes:

• Webhook handling + callback token verification
• Error handling with common error codes
• Common mistakes and anti-patterns
• Idempotency key guidance for payouts

Xendit is the leading payment infrastructure provider in Southeast Asia (Indonesia, Philippines, Malaysia, Vietnam, Thailand).

SDK source: https://github.com/xendit/xendit-node