[Docs] Site structure

## Summary
Set up developer documentation site structure.

## Options
- Mintlify (hosted)
- Nextra (Next.js based)
- Custom MDX

## Sections
- Getting Started
- API Reference
- Authentication
- MCP Integration
- Webhooks
- SDKs

## Acceptance Criteria
- [ ] Docs framework chosen
- [ ] Navigation structure defined
- [ ] Search working
- [ ] Code examples supported