Document vanilla OpenAPI and x-kusk enabled configuration files merging

[Tarick]

Describe the enhancement you'd like to see

OpenAPI files are frequently generated from the code, where x-kusk extention information is not available.
Even with the plan-first approach there is usually only one OpenAPI file while multiple testing and production environments exist that have different x-kusk values. Supporting separate OpenAPI file with x-kusk per env is cumbersome during API development.
I'd like to have the documented way to merge vanilla OpenAPI file and file with the x-kusk extention into one that can be fed to Kusk for generation, i.e. there could be one OpenAPI file and multiple (one per env) x-kusk enabled YAML files.