Generate OpenAPI specification

[php-coder]

We can just generate a file or generate and serve it as a static file.

[php-coder]

Some useful links to play with:

• https://openapi.tools/
• https://editor.swagger.io/
• https://www.apibldr.com/
• https://mermade.github.io/openapi-gui/ (https://github.com/Mermade/openapi-gui)
• https://interaction-design.co.za/openapidesigner/