Add use-cases to the "Available endpoints to extend" documentation

[opr]

In the https://github.com/woocommerce/woocommerce-blocks/blob/trunk/docs/third-party-developers/extensibility/rest-api/available-endpoints-to-extend.md documentation, we list all the endpoints we have that can be extended, however it would be great to add use-cases to this list and explain why someone might need to extend each endpoint. Currently this list doesn't add much value on its own.