This commit updates `docs/instance-config.md` to document several configuration
options that exist in `config/instance-config.js` but were missing from the
documentation:
- `LOCKDOWN_TITLE` and `LOCKDOWN_MESSAGE`: UI text for maintenance mode.
- `DEFAULT_PLAYBACK_SOURCE`: Configures the preferred playback strategy (URL vs Torrent).
- `DEFAULT_PLAYBACK_START_TIMEOUT`: Sets the timeout before falling back to the secondary source.
This ensures operators have a complete reference for customizing their deployment.
Co-authored-by: PR0M3TH3AN <[email protected]>