Add sub resource support in RepresentationModel

[toedter]

The current RepresentationModel only supports links. Since many concrete representations support sub resources, e.g.:

• _embedded in HAL / HAL Forms
• entities in Siren
• included in JSON:API

it would be nice if this concept is supported in the generic representation model, so it could be mapped automatically if this concept is supported by a concrete representation.

This is related to #864 and #968

[odrotbohm]

We took a first stab at a HAL specific model builder with 2cab91a. See the updated reference documentation for details.

[ccit-spence]

The current documentation does not show how this would work for a PagedModel. Is this something that can be documented? Are there current examples?