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.

Commencez par examiner la terminologie et les noms proposés dans cette issue, ainsi que la discussion associée dans #225. La tâche sera considérée comme terminée lorsqu’une terminologie aura été convenue et que des mises à jour cohérentes auront été effectuées partout où le projet documente ou expose ces concepts, mais les fichiers concernés et l’étendue exacte ne sont pas indiqués ici.

Rédigé par le modèle d'indexation à partir du texte de l'issue.