Fix reference link text/agent logs to Agent and standardize default port mappings in compose

[eduardomozart]

This PR introduces minor fixes to the documentation and the deployment example to improve clarity and user experience for new deployments.

Changes

1. README.md Improvements:

• Fixed a reference typo: Corrected the link label for the Agent configuration section which was incorrectly named netxmsd.conf instead of nxagentd.conf.
• Actionable Troubleshooting: Added a specific Docker command (docker compose logs agent) to the connection problems section, making it easier for users to debug agent-to-server communication issues without searching for log locations.

2. Compose Example Standardization:

• Port Mapping: Changed the external ports for the server service from 48920:4701 and 48921:4703 back to the standard NetXMS ports 4701:4701 and 4703:4703. The current non-standard port mapping (48920) often leads to "Connection Refused" errors when users try to connect using the NetXMS Management Console (Rich Client) for the first time, as the desktop application defaults to port 4701. Standardizing these ports in the deployment-example follows the principle of least surprise and aligns with the official port documentation, significantly reducing initial setup friction for new users.