When adding the Stroom service user, ensure its a system user

As Stroom runs as a system service, it's user id should be a system one. In the documentation when the Stroom service user is created, it should be created as a system user.

This way standard user auditing postures from Linux auditd (e.g. monitoring file deletes) will not result in excessive auditd events during normal Stroom operation.

Find the documentation section that explains creating the Stroom service user and check the command or instructions used there. Update that step so the account is created as a system user, then verify the surrounding installation instructions still accurately describe the service setup.

Written by the indexing model from the issue text.