Noves-Inc/canton-data-app: docker-compose/migrate_to_v3_12.md

# Docker Compose Migration Guide – Adding Multi-Validator Support (v3.12.0+) This guide covers how to add a second (or more) Canton validator node to an existing single-node deployment after upgrading to v3.12.0 or later. If you are deploying for the first time, follow [docker_compose_deployment.md]