Improve OpenAPI (REST) documentation for the website with a docusaurus plugin

[rikoe]

The App Directory documentation uses a generated ReDoc static html file for the API docs, based on appd.yaml, but docusaurus now have several nice plugins for integrating OpenAPI documentation which we should incorporate.

One of them is https://github.com/cloud-annotations/docusaurus-openapi (example: https://docusaurus-openapi.netlify.app/).

The other is https://github.com/rohit-gohri/redocusaurus (example: https://redocusaurus.vercel.app/).

[kriswest]

Currently, the docs are rendered by a static html file that dynamically renders the current spec file: https://github.com/finos/FDC3/blob/master/website/static/schemas/next/app-directory.html
(my inner pedant is an extrovert)

That said, the docusaurus plugins clearly do a better job of the integration with the site - particularly if they can get it into the versioned sidebar nav (where currently the full spec is a static file that opens in a new window).

Is this something you're up for/have time to work on? or do we add it to the queue?