fix relations docs to match erp-integration-api source of truth

Replace fabricated relation (singular) / source_field format with the
correct relations (plural) / items / unique_ids object format from
MAPPING_V2.md. Remove non-existent properties (relation_type,
source_field), fix enabled placement, add _tags, operation examples
with before/after states, relation references, and _id preservation.

Author: Karl Rankla