Docs: Model reference documentation should talk about "why / what is a model"

[coco98]

On the model documentation https://hasura.io/docs/3.0/supergraph-modeling/models shouldn't we mention why we add models?

For example:

Models create a standard API with a, b, c features and compose well with each other. here's an example of a model actually shows up in your API.

And then we go into the reference documentation?

[seanparkross]

https://github.com/hasura/v3-docs/pull/501