Define the ISO mdoc credential format-specific protocol details

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. The format itself is ISO-owned, so the work here is how this protocol uses it.

Use the same headings, in the same order, as #22 (SD-JWT VC), so the two sections (SD-JWT VC and ISO mdoc) 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

Structure still to settle, and it applies to both format sections: one appendix per format, or format-specific sub-sections inside each protocol section? An appendix lifts out into a separate document more cleanly, which is what we agreed to keep cheap, so that would be my preference.