[did2 #14] Migration guide (user-facing docs)

[stevevanhooser]

Part of the NDI-matlab did2 migration. Target branch: Vnext.

Scope

One-page user-facing migration guide explaining what changed, how to upgrade, and what breaks.

Suggested location: docs/MIGRATION_TO_V_DELTA.md (or whichever docs/ convention the repo uses).

Contents:

• What V_delta is and why we're migrating (link to did-schema/schemas/V_delta_SPEC.md).
• What stays the same: the doc.document_properties.<class>.<field> accessor pattern, ndi.query signature, ndi.database API, base.* field names.
• What breaks (the small list):
  • The four collapsed-ontology classes: probe_location, treatment, ontology_image, ontology_label. Show before/after snippets for each.
  • depends_on(k).id → depends_on(k).value (mitigated by compat; document the canonical V_delta form).
  • Removal of document_properties.ndi_document.* (long-deprecated; document the move to base.*).
  • Removal of document_class.superclasses[i].definition (internal plumbing; affects any user code that read it directly).
• How to migrate:
  • Local: ndi.migrate.local(path).
  • Cloud: ndi.migrate.cloud(datasetId) — including the publish-state flow and the ForceUnpublish flag.
• Deprecation timeline (see issue 15).

Out of scope

API reference docs. This is a guide, not a manual.

Dependencies

Rough draft can start after issue 5 (alias table makes the "what changed" list concrete). Finalize after issue 10 (so usage examples are accurate).