Update README.md documentation for new persistence structure from PR #117

[michaelstingl]

Description

PR #117 introduced significant changes to the persistence configuration structure, but the documentation in charts/opencloud/README.md still shows the old format.

What needs to be updated

1. OpenCloud persistence configuration

The README currently shows:

| `opencloud.persistence.enabled` | Enable persistence | `true` |
| `opencloud.persistence.size` | Size of the persistent volume | `10Gi` |
| `opencloud.persistence.storageClass` | Storage class | `""` |
| `opencloud.persistence.accessMode` | Access mode | `ReadWriteOnce` |

Should be updated to reflect the new structure:

• Split into config and data sections
• Add existingClaim documentation

2. Add documentation for existingClaim feature

Need to document the new existingClaim parameter for:

• postgres.persistence.existingClaim
• onlyoffice.persistence.existingClaim
• opencloud.persistence.config.existingClaim
• opencloud.persistence.data.existingClaim
• opencloud.storage.s3.internal.persistence.existingClaim

3. Add example usage

Show how to use existing PVCs:

opencloud:
  persistence:
    data:
      existingClaim: "my-existing-data-pvc"
    config:
      existingClaim: "my-existing-config-pvc"

Related

• Introduced in PR Allow use of existing PVCs in values.yaml #117

cc @teo - Since you implemented these changes, would you be able to help update the documentation? 🙏