Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Reference to Existing Issue

Closes #611.

Description of Changes

This PR adds a new section explaining the default alert settings in OpenWISP Monitoring. The updated documentation includes:

• A new Default Alert Settings section listing predefined alerts.
• A description of common default alerts such as Ping Loss, High Latency, Low Disk Space, High CPU Usage, and Iperf3 Bandwidth Drop.
• Clarification that these alerts are customizable via the device page.

These changes enhance the documentation by making it easier for users to understand the built-in monitoring alerts and their behavior.

@nemesifier kindly review it