This is a quick and dirty way to build you first APIs.json file. Most likely you came across one of my tools that needed an APIs.json file to do something interesting with your OpenAPI Spec. All you need to create is add the URL for your OpenAPI Spec, and this will do the rest.
This application runs as a micro tool, which operates 100% client-side, running completely on a Jekyll website, hosted using Github, and Github pages. Visit the about page for more information, if you'd like to support this work feel free to invest and done. If you have any issues, or would like to leave a comment, you can do so via the project's Github repository.