Our terminology around documents, nested, and types is confusing

This is coming from #225. All of these things are different within the context of Contentlayer:

- _Document_ is a generated piece of content.
- _Document Type_ is a group of content of a similar shape.
- _Nested Type_ is a repeatable group of content using within document types
- _Type_ is an auto-generated TS type for every document and its nested types.

Aside from the nuance of difference between "document" and "nested" (covered in #225), I'm finding our use of "type" to clash with our heavy use of TypeScript and in auto-generating TS type definitions.

If we were to pursue #225, we could simplify our terminology like so:

- _Model_ or _Content Type_ is a group of any type of content.
- _Document_ is the generated data file.
- _Type_ is the generated TS definition.

Example: A `Post` model defines the shape of content in the `content/posts` directory. Contentlayer processes this content according to the model definition, and then generates a _document_ for every post, along with a TS _type_ definition.