Add project documentation

docs/sensors.md

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+# Sensor Architecture
+
+## Current Sensors
+
+### Fridge Zone 1
+
+Purpose:
+Refrigerator or freezer monitoring.
+
+Alarm:
+User configurable.
+
+### Fridge Zone 2
+
+Purpose:
+Refrigerator or freezer monitoring.
+
+Alarm:
+User configurable.
+
+### Rear Seat Area
+
+Purpose:
+Monitor child seating area.
+
+Alarms:
+- Warning Threshold
+- Critical Threshold
+
+### Outside Air
+
+Purpose:
+Ambient outside temperature.
+
+## Future Sensors
+
+### Battery Bay
+
+Purpose:
+Battery compartment monitoring.
+
+### Cabin Air
+
+Purpose:
+Cabin temperature monitoring.
+
+### Water Tank
+
+Purpose:
+Water freeze protection.
+
+### Cooler
+
+Purpose:
+Secondary food storage monitoring.
+
+## Sensor Bus
+
+DS18B20 OneWire
+
+Single bus shared across all sensors.