Add Fumadocs documentation site with auto-generated API reference

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Summary

• Adds a complete documentation site built with Fumadocs (Next.js) in docs/
• 75 pages covering Getting Started, Guides, API Reference, and Architecture
• API Reference (62 pages) auto-generated from a 3500-line OpenAPI 3.1 spec via fumadocs-openapi
• Beautiful default theme with dark mode, full-text search, and responsive layout

Content

Section	Pages	Description
Getting Started	3	Installation, Quick Start, Connecting the SDK
Guides	5	Webhooks, Test Clocks, 3DS, Dashboard, Idempotency
API Reference	62	Auto-generated from OpenAPI spec for all 13 resources
Architecture	4	Overview, Request Lifecycle, Services, Contributing

Usage

cd docs
bun install
bun run dev       # Dev server at localhost:3000
bun run build     # Production build
bun run generate  # Regenerate API docs from openapi.json

Test plan

• bun run build succeeds (75 static pages generated)
• All 13 API resources have generated reference pages
• Visual check: bun run dev and browse all sections