Consider removing the distinction between document and nested types

From the beginning, we've separated the idea of _documents_ and _nested_ types. The primary difference between these types is that:

- Document types define the shape of built content files
- Nested types define shapes of content used _within_ built content files

We may be able to simplify the API by considering every type definition as just that — a type definition. We already have to define [`documentTypes`](https://www.contentlayer.dev/docs/reference/source-files/make-source#documenttypes) when calling `makeSource`. Could this be the way in which documents are defined, while every other type is just a type.

#226 contains a way we could adjust terminology if making this change to help with the language we're using.