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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
If you're explicitly about API docs, Redoc is pretty good if you write your API docs down in the OpenAPI format. It's also autogenerated, though.
There are lots of good solutions here. Docusaurus is one I prefer because the docs are written in markdown, but extensible with React, and can be versioned. However, you have to implement your site within the Docusaurus framework instead of embedding it into an existing site. (I can't find any documentation on inverting things like that, at least.)