#32: 3.0, 3.1, 3.2

[plutov]

finally, the decision was to use libopenapi

Summary by CodeRabbit

• New Features
  • Expanded OpenAPI compatibility to all 3.x versions (3.0, 3.1, 3.2) for broader spec support.
• Documentation
  • Updated README to reflect expanded OpenAPI support and added notes to contributing guidelines.
• Examples
  • Streamlined example API specs; removed an outdated 3.1 example and simplified the primary YAML to a production-focused contract.
• Tests
  • Overhauled tests to run against example files, adding coverage for navigation and empty documents.
• Refactor
  • Migrated to a new OpenAPI engine for more consistent parsing and rendering.
• Chores
  • Updated dependencies.