docs: add Tigris as S3-compatible storage backend, fix s3 region field name (#15361)

## Summary

Add Tigris configuration to the Configuration and Backup & migration
pages, using the existing AWS_S3 backend — no code changes required.
Fix `region` → `region_name` in the existing S3 config example in
`backup_and_migration.md`. The code in `s3_conn.py` reads `region_name`,
so the previous field name was silently ignored.

##Context

With MinIO's open-source repository archived (#13840 on
infiniflow/ragflow), users need documented alternatives for object
storage. Tigris is S3-compatible and works with RAGFlow's existing
AWS_S3 backend out of the box.

## Changes

`configurations.md`: Added `### s3 (Tigris)` section after `### minio`,
matching the existing reference style. Includes config block, field
descriptions, and a pointer to `service_conf.yaml.template` for other
S3-compatible backends.
`backup_and_migration.md`: Added Tigris config block under single-bucket
mode. Fixed region → region_name in the existing S3 example. Added
Tigris to the supported backends list.

##Notes

No new files — edits to existing docs only.
Config field names (`access_key`, `secret_key`, `region_name`,
`endpoint_url`, `bucket`, `prefix_path`, `signature_version`,
`addressing_style`) verified against `rag/utils/s3_conn.py`.