How to add custom endpoints to generated OpenAPI docs?

[Sitin]

Case: I want to add my custom routes to generated API specification (i.e. docs). Basically, I want to see my generic Symfony routes in the API documentation. How can I do that?

These are the routes for integration with 3rdparty services, health checks and so on. They don't have underlying entities by nature. I know, that I can fake such entities but this looks awkward and requires to break the simplicity of Symfony.