Use [[def: ]] and [[ref: ]] tags to define and reference data models

[swcurran]

Use the [[def: ]] and [[ref: ]] tags in the markdown to define data models and reference them elsewhere in the specification.

To start, find an appropriate reference and try out the approach. If found to be effective, establish a list of other places to use this, and apply the technique elsewhere in the specification.

This is an action from the AnonCreds Specification Working Group meetings - 20220613 and 20220627.