Add a simple openApi-3 specifition for Jenkins-rest

[Lordfiber]

Hello

I am an applicant for Google Summer of Code There is a proposal in the Jenkins community to automatically generate OpenAPI-3 documentation for our Jenkins REST API.

Here is a manually crafted OpenAPI documentation for the JobInfo() method in Jenkins-rest, hoping it complies with the standards. I will continue to update it in the future.

openapi-3.md

[cdancy]

@Lordfiber that link is broken for me ... but yeah feel free to do all of the stuff :)