Docs: Integrate Fumadocs + OpenAPI #13

Description

Set up documentation site with Fumadocs and integrate OpenAPI specification.

Tasks

• Generate/maintain OpenAPI spec for exec0 API
• Set up Fumadocs
• Integrate OpenAPI spec with Fumadocs
• Render API reference documentation
• Enable try-it-out experience via Fumadocs/OpenAPI integration
• Set up docs deployment pipeline

Documentation Sections

• Getting Started
• Authentication
• API Reference (auto-generated from OpenAPI)
• Code Examples
• Pricing/Billing explanation

Acceptance Criteria

• API documentation is auto-generated from OpenAPI spec
• Users can try API calls directly from docs
• Documentation is versioned and deployable
• Spec stays in sync with implementation