Add LibreTranslate as an optional scenario in the Docker Compose stack #607

As a system administrator, I want to add LibreTranslate to my Docker Compose stack via a predefined scenario, so that I can self-host a translation service for Zammad without manually configuring the container myself.

Acceptance criteria

• The Docker Compose setup offers a LibreTranslate scenario/profile that can be enabled with minimal configuration
  • Enabling it should not require manual editing of networking or volume definitions
• The LibreTranslate service starts correctly alongside the existing Zammad stack
• Documentation describes how to enable the scenario and how to connect Zammad to the resulting LibreTranslate instance