Feat: Add the migration script and its doc, added `backup` as default…

[Treedy2020]

What problem does this PR solve?

This PR adds a data backup and migration solution for RAGFlow Docker Compose deployments. Currently, users lack a standardized way to backup and restore RAGFlow data volumes (MySQL, MinIO, Redis, Elasticsearch), which is essential for data safety and environment migration.

Solution:

• Migration Script (docker/migration.sh) - Automates backup/restore operations for all RAGFlow data volumes
• Documentation (docs/guides/migration/migrate_from_docker_compose.md) - Usage guide and best practices
• Safety Features - Container conflict detection and user confirmations to prevent data loss

Type of change

• [x] New Feature (non-breaking change which adds functionality)
• [x] Documentation Update