While is a cool idea on its own, I don't get why they try to reinvent it as a new system. We've got swagger, openapi, graphql and many other systems that already describe the APIs. They mostly include documentation too. Why not just expose those for the same effect? (If I was cynical, I'd guess Vercel wanting a proprietary thing of their own just for less portability)
those describe server APIs
how would it relate to ui?
OpenAPI is great, there are a lot of tools to go from OpenAPI to UI, but you don't have a lot of control over the presentation.
For instance, you can specify that "first_name","last_name" and "email" are strings, but not that first/last name should be next to each other and email in its own row.
There are supersets of OpenAPI that can control the look and feel more. JSON Forms, for instance.
This is cool, too, though.