feat: adding API design guidelines (#137)

### TL;DR

Added API design guidelines documentation to standardize our API development practices.

### What changed?

Created a comprehensive `README.md` in the `openapi` directory that outlines best practices for API design, including:

- Directory structure and file naming conventions
- Versioning strategy and breaking change guidelines
- Naming conventions for resources, IDs, and fields
- Service patterns for states, actions, and resources
- OpenAPI best practices for documentation and validation
- HTTP methods and status code usage
- Pagination implementation details
- Error handling structure and guidelines
- Stainless SDK patterns and examples
- Redocly CLI usage for bundling and linting

Author: pengying