Add example of why and how to use delegated constraints in inheritance constraints docs

[scotttrinh]

We have excellent fully fleshed out examples of how inheritance works in constraints on the constraints documentation (see https://www.edgedb.com/docs/datamodel/constraints#constraints-and-type-inheritence), but the example in the inheritance documentation is very sparse and doesn't even currently link to the constraints documentation (see https://www.edgedb.com/docs/datamodel/inheritance#ref-datamodel-inheritance-constraints).