Add guidance on `did:webvh` in VCP documentation.

Impacted Sections

• https://spec-untp-fbb45f.opensource.unicc.org/docs/specification/VerifiableCredentials#did-methods

Issue Description

The new did:webvh method (did:web + Verifiable History) extends did:web by adding verifiable history of DID Document updates, self-certifying identifiers, and optional features like key pre-rotation and witnesses. It remains backwards compatible with did:web.

Why this matters for UNTP: While VCP currently references did:web, it may be useful to consider including guidance on did:webvh in the specification. This would help implementers understand the potential benefits and tradeoffs, especially in higher-assurance supply chain scenarios where persistence and auditability of identifiers are important.

I Have

• Added the relevant labels (e.g. WG-Adoption, WG-Conformity, WG-SupplyChain, WG-Technical, WG-Steering, etc.).