refactor: remove duplicate MCP service log files

Remove redundant log file creation for MCP services since output is
already captured by Docker logs. This simplifies deployment by removing
unnecessary volume mounts and file management.

Changes:
- Remove logs/ directory creation from Dockerfile
- Remove logs/ volume mount from docker-compose.yml
- Update start_mcp_services.py to send output to DEVNULL
- Update documentation to reflect changes (DOCKER.md, docs/DOCKER.md)
- Update .env.example to remove logs/ from volume description

Users can still view MCP service output via 'docker logs' or
'docker-compose logs -f'. Trading session logs in data/agent_data/
remain unchanged.

docker-compose.yml

@@ -7,7 +7,6 @@ services:
     container_name: ai-trader-server
     volumes:
       - ${VOLUME_PATH:-.}/data:/app/data
-      - ${VOLUME_PATH:-.}/logs:/app/logs
       # User configs mounted to /app/user-configs (default config baked into image)
       - ${VOLUME_PATH:-.}/configs:/app/user-configs
     environment: