docs: add official OpenAPI specification v1.9.0

This specification serves as the authoritative reference for:
- Build API structure and patterns
- Available BuildAction types and parameters
- Part types (FilePart, HTMLPart, NewPagePart, DocumentPart)
- Output format options (PDF, PDF/A, images, Office, JSON)
- Error response schemas
- Authentication methods

Will be used to ensure our implementation follows the official API design.

Author: jdrhyne