docs(demo): document shared storage, voting devices and release hold

[sqlrush]

Documentation-only follow-up to #82, within the same container delivery.

• Explain the single-host shared filesystem, generated directory layout and distinct per-node PGDATA/WAL.
• Document three voting backing files, exact loop-device mappings, fixed capacity, direct I/O checks and permissions.
• List effective database settings, host preparation, read-only post-install checks and normal stop/restart behavior.
• Link the storage chapter from the MVP entry and retain the visible image publication hold.

Verification: 17 Bash command blocks passed bash -n; relative document links and whitespace checked. Host preflight, generated metadata/configuration and filesystem/permissions were checked read-only on the preserved CLEAN rehearsal6 scene; no restart, workload or kernel/configuration change.

The known publication failure in run 35500993944 is NOT fixed or waived by this documentation change. The image remains unpublished. The workflow may automatically run native smoke for this path, but a later PASS cannot erase the unresolved shutdown failure.