[did2 #774c] Deprecate src/ndi/ndi_common/schema_documents/ in favor of the DID-schema loader

[stevevanhooser]

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

Scope

Once the schema loader from #774 is in place and pointing at DID-schema's V_delta/stable/ (via local cache + fallback bundle), the bundled directory src/ndi/ndi_common/schema_documents/ is redundant.

• Remove src/ndi/ndi_common/schema_documents/ from the repo (the fallback bundle from [did2 #774b] NDI schema loader: fetch from DID-schema V_delta, cache locally, fallback bundle #774 lives elsewhere — likely src/ndi/ndi_common/schemas_fallback/ or similar).
• Update any code path still reaching into the old directory directly to go through the schema loader instead.
• Update build/packaging steps that copied the old directory.
• Update docs that referenced the old path.

Out of scope

The loader itself — that's #774.

Dependencies

• [did2 #774b] NDI schema loader: fetch from DID-schema V_delta, cache locally, fallback bundle #774 must land first (the loader needs to be the only schema source before we can delete the bundled set).
• Possibly the three missing classes (animalsubject, subjectmeasurement, treatment_transfer) need to be resolved in DID-schema first so deletion doesn't lose functionality. Confirm before landing this issue.