Define the SD-JWT VC credential format-specific protocol details

[awoie]

Split out of #8 so the per-format sections can be worked in parallel.

Per the 2026-07-20 discussion we define the formats (or at least the profiles) in this WG, and we write the text as if it will become a separate document later, so detangling stays cheap. SD-JWT and SD-JWT VC are already at IETF, so the work here is how this protocol uses them, not redefining the format.

Use the same headings, in the same order, as #21 (ISO mdoc), so the two sections stay uniform.

• Value of the credential format identifier
• Content of the credential type field
• Meaning of the path element and how it relates to specific fields in the
  credential / response
• How value matching works
• How the issuer identifier request applies
• Format specific extensions (e.g. cryptographic algorithm values)
• How the cryptographic binding parameter request applies
• Structure of the response for this format
• What the transaction data request means for this format
• How the transaction transcript is included in the response

Use the same headings, in the same order, as #21 (ISO mdoc), so the two sections stay uniform. Overall structure (appendix vs per-section sub-sections) is being settled in #21; follow whatever lands there.