Our terminology around documents, nested, and types is confusing

[seancdavis]

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.

[iamnbutler]

Huge +1 on moving away from using Type anywhere. I think that would be a really positive change.