OCI Object Storage as an S3-compatible backup destination

## Summary

I would like to contribute documentation for using Oracle Cloud Infrastructure
(OCI) Object Storage as an S3-compatible backup destination with the Barman
Cloud CNPG-I plugin.

This is a documentation/configuration contribution. It does not add a native
OCI-specific provider implementation.

## What was validated

- Continuous WAL archiving
- On-demand physical backup
- Scheduled backups
- Backup object validation in OCI Object Storage
- Restore/bootstrap of a new CloudNativePG cluster from the stored backup
- Recovery readiness / WAL replay

## OCI configuration

- OCI Object Storage S3 Compatibility API
- Endpoint pattern:
`https://.compat.objectstorage..oci.customer-oci.com`
- OCI Customer Secret Key used as S3 access key and secret key
- Explicit OCI region configured for request signing
- Unique destination path per PostgreSQL cluster

## Validated versions

| Component | Validated version / image |
|---|---|
| CloudNativePG Operator | 1.29.1 |
| Operator image | `ghcr.io/cloudnative-pg/cloudnative-pg:1.29.1` |
| PostgreSQL | 18.3 |
| PostgreSQL image | `ghcr.io/cloudnative-pg/postgresql:18.3-system-trixie` |
| PostgreSQL build | `postgres (PostgreSQL) 18.3 (Debian 18.3-1.pgdg13+1)` |
| Barman Cloud Plugin | 0.12.0 |
| Plugin image | `ghcr.io/cloudnative-pg/plugin-barman-cloud:v0.12.0` |
| Detected cluster plugin | `barman-cloud.cloudnative-pg.io`, version `0.12.0` |

## Proposed documentation

I can submit a concise OCI Object Storage provider guide containing:

1. OCI prerequisites and IAM guidance
2. Kubernetes Secret example using placeholders
3. `ObjectStore` manifest
4. Cluster WAL-archiving configuration
5. On-demand and scheduled backup examples
6. Recovery example
7. OCI-specific troubleshooting and security notes

Reference implementation and test details:
https://medium.com/oracledevs/cloudnativepg-oci-object-storage-a-practical-backup-and-restore-guide-98d35c204b55

Could you please confirm whether an OCI Object Storage configuration guide would be welcome as part of the Barman Cloud Plugin documentation?

首先检查现有的 Barman Cloud Plugin 文档结构，以及本 issue 中描述的已验证 OCI 配置。添加一份简明的 provider 指南，涵盖前置条件、Secret 和 ObjectStore 示例、WAL 归档、备份、恢复、故障排除和安全注意事项；当文档化的工作流与列出的已验证场景一致时，即视为完成。

由索引模型根据 Issue 内容生成。