Add Network UPS Tools docs

[BeeJoe]

Summary

Adds StartOS documentation for Network UPS Tools (NUT).

• Adds a new System docs page for NUT configuration.
• Documents disabled, direct UPS, and network UPS client modes.
• Explains UPS status output, refresh behavior, troubleshooting checks, and safe testing.
• Adds the page to the StartOS sidebar and index between WiFi and Internationalization.
• Updates the existing Surge Protector and UPS page to point to NUT-based automatic shutdown support.

Related StartOS PR

Corresponding implementation PR: Start9Labs/start-os#3317

Validation

• git diff --check
• mdbook serve -p 3001 --hostname 127.0.0.1 built and served the StartOS book locally. The build emitted pre-existing HTML tag warnings from backup-create.md, unrelated to this change.