Generate default documentation for registered routes

The old plugin had support for default responses:

```
OpenApiOptions().apply {
defaultDocumentation { doc ->
doc.json("500", ErrorResponse::class.java)
doc.json("503", ErrorResponse::class.java)
}
}
```

These are responses which could occur on any endpoint.

Start by finding where registered routes are converted into OpenAPI documentation and how responses are represented. Use the old plugin example as the behavioral reference; done means configured default responses such as 500 and 503 appear on every endpoint without requiring per-route declarations.

Written by the indexing model from the issue text.