Migrate Appendix C. "Subject-Holder Relationships" to VC Implementation Guide

[msporny]

The creation of Appendix C. "Subject-Holder Relationships" predated the creation of the VC Implementation Guide and contains material more suited for the Implementation Guide rather than the core specification.

Given that the VC2WG is going to focus on modularizing the specification a bit, and (ideally) reducing the size of the core specification to aid readability, and for the additional reason above, we should migrate Appendix C. "Subject-Holder Relationships" to the VC Implementation Guide.