[Nexthop] Documentation for debugging utilities

[anna-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

This PR adds comprehensive debugging documentation for FBOSS when the CLI is not available, typically when the hardware agent (fboss_hw_agent) is down or not responding.

The new documentation covers:

Service Status and Logs

• Checking FBOSS service status with systemd
• Viewing service logs with journalctl
• Additional log file locations (boot history, snapshots)
• Service dependency tree

Crash Dumps and State Files

• Crash dump locations and analysis
• Core dump management with coredumpctl
• Using gdb for debugging

Direct Hardware Access Utilities

• wedge_qsfp_util - Transceiver debugging with --direct-i2c
• weutil - EEPROM information
• fw_util - Firmware versions
• fixmyfboss - Automated diagnostic tool
• showtech - System information collection with --details options
• Other diagnostic utilities

Important Runtime Files and Directories

• Configuration files
• Platform information (including dmidecode for system info)
• Warm boot state
• SDK dumps

Debug Builds

• Building with debug symbols
• Using debug builds with gdb

GDB Debugging

• Attaching to running processes
• Debugging core dumps

Common Troubleshooting Scenarios

• Hardware agent won't start
• Service crashes immediately
• No logs available

Test Plan

• Verified all commands on actual FBOSS device
• Tested documentation rendering in Docusaurus dev server
• Confirmed all internal links work correctly
• Validated that generic examples don't expose real hardware identifiers