Extend documentation for adding custom volumes to agent containers

[nezhar]

Summary

Extend the documentation to explain how users can add custom volumes to an agent container.

Scope

• Document the supported configuration for defining custom volume mounts.
• Explain how host paths, container mount paths, and any relevant options are specified.
• Include a practical configuration example.
• Note any constraints, permissions, security considerations, or platform-specific behavior.

Acceptance criteria

• Documentation includes clear, step-by-step guidance for adding custom volumes to an agent container.
• At least one complete example is included.
• Limitations and expected behavior are documented.