docs: "BuiltIn APIs" page, title and layout suggestion.

[RobertKielty]

https://docs.kcp.io/kcp/main/concepts/apis/built-in/#built-in-apis

I think the flow of Built In APIs page might be improved by moving the last paragraph to the top of the page and by being even more explicit about the origin of the listed API endpoints.

I think also it might be an good idea to re-title this page to indicate that the list of APIs presented are the Kubernetes API Endpoints that kcp uses.

At the moment a new kcp user might reasonably expect to see API Endpoints that are defined by kcp not the endpoints that are inherited from Kubernetes API. The title BuiltIn APIs is imo vague.

Does kcp have a swaggerfile that provides definitions of the provided APIs would it be possible to reference detailed docs for all Endpoints?

Can we add one-liners to describe why and how kcp is making use of the groups od k8s api endpoints? That would be useful for new kcp end-users to understand how kcp leverages Kubernetes.

@embik I've made a first pass at sketching this out. Keen to know your thoughts, PR on the way.

[embik]

Does kcp have a swaggerfile that provides definitions of the provided APIs would it be possible to reference detailed docs for all Endpoints?

Since it's all Kubernetes-compatible, kubectl api-resources or other OpenAPI v3 discovery tools can be used to review APIs. The main reason against providing detailed API schemas is: a) they are identical to Kubernetes and b) every workspace has a different set of APIs. Providing OpenAPI v3 specs as static artifact in our docs would seem like that is all the APIs available, which isn't true.