[MCC-105863] RepresentorBuilder documentation

This PR:

* Adds xmldoc info for ```IRepresentorBuilder```. The docs aren't added to the concrete class because consuming apps should be coding against the interface, not the concrete implementation. Intellisense will show when coding against ```IRepresentorBuilder```.
* Added representor_builders.md doc with sample code

@mdsol/hypermedia-team Please merge ASAP.