Feat: Add the migration script and its doc, added backup as default… (#8245)

### 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

Co-authored-by: treedy <treedy2022@icloud.com>

.gitignore

@@ -193,3 +193,5 @@ dist
 # SvelteKit build / generate output
 .svelte-kit
 
+# Default backup dir
+backup