Missing documentation for `network.proxy` field in configuration file

[cmaglie]

Describe the problem

The documentation for network.proxy field in configuration file is missing.

To reproduce

N/A

Expected behavior

N/A

Arduino CLI version

nightly

Operating system

N/A

Operating system version

N/A

Additional context

No response

Issue checklist

• I searched for previous reports in the issue tracker
• I verified the problem still occurs when using the nightly build
• My report contains all necessary details