Add Body & Response Examples

Missing `body` and/or `response` examples on many of the key endpoints.

E.g. `Items`:
- missing body payload examples for simple and relational payloads
- missing response examples for simple and relational responses

Understanding that the dynamic and agnostic nature of Directus makes this difficult, it is still extremely helpful to see the structure of the body and responses via an example.

Perhaps 1 or more example use cases could be used to demonstrate the usage of the endpoints:
- simple CMS webpage
- simple Product Management
- could align with templates available on cloud?
- if more than one use case option, include a way to choose the example

Just some ideas. Mostly need examples similar to the older API docs.