docs: add embedding documentation to usage/setup section

[Copilot]

Adds documentation for embedding Z-Wave JS UI in other software (e.g., Home Assistant add-ons) where settings are managed externally via environment variables.

Documentation Added

• Controller Port Override (ZWAVE_PORT): Force-enable Z-Wave and override configured port, disables serial port enumeration in UI
• External Settings Management (ZWAVE_EXTERNAL_SETTINGS): Path to JSON file with externally managed driver/server settings, hides corresponding UI controls
• Supported External Settings: Complete reference for all configurable settings:
  • Logging (UI + driver-only options)
  • RF configuration (region, auto power levels)
  • Storage settings (cache dir, throttle mode)
  • Security keys (S0, S2, Long Range)
  • Z-Wave JS Server configuration
  • Driver presets
• Example Configuration: Real-world Home Assistant add-on setup with JSON structure

External Settings File Example

{
  "logEnabled": true,
  "logLevel": "info",
  "rf": {
    "region": 0,
    "autoPowerlevels": true
  },
  "securityKeys": {
    "S0_Legacy": "0102030405060708090A0B0C0D0E0F10",
    "S2_Authenticated": "2122232425262728292A2B2C2D2E2F30"
  },
  "serverEnabled": true,
  "serverPort": 3000
}

Documentation placed at end of docs/usage/setup.md as requested.

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[zwave-js-assistant]

💡 The pull request title does not match our guidelines 💡

We follow conventional commits, check out the description and examples. This means that the PR title must...

• start with the type of change, e.g. feat:, or fix:
• optionally specify a scope, for example we use config for changes to config files: feat(config):
• contain a description that explains what is being changed, e.g. add a parameter, correct a typo, ...
• and ideally explains what the change affects, e.g. the device (manufacturer, device label, firmware, ...)

The description should use uppercase and lowercase normally, except for the first character which must be lowercase.