Confluence: Test and document object storage integration

One of the BBTOC requirements of moving into the Incubating phase is integration with object storage.

Confluence supports S3-compatible object storage for storing attachments.

The chart supports object storage configuration in the values.yaml, but this functionality should be tested and verified working.

The integration should be tested with both S3 and an internal MinIO cluster deployed via the Big Bang addon.

Once the integration is validated as working, both scenarios (S3 and MinIO) should be documented in confluence/docs/.