Add reference to living threat model document

[msporny]

From the SING review:

Threat model – acknowledged WG efforts and suggested referencing a living, community-maintained model.
Add a short pointer section, e.g., “Threat Modeling Resources for DID Deployments,” linking to the evolving model.
Encourage iterative contributions and updates aligned with new use cases. This improves implementer readiness and continuity beyond the current charter.

[msporny]

A PR should be raised that points to a WG maintained threat model that is a living document.