This PR introduces a new attribute, soilPh, to the AgriParcelRecord data model. The new property is designed to support the recording of measured soil pH values (recommended scale 0–14), a common requirement for modern soil monitoring and precision agriculture scenarios.

Motivation:

• The current AgriParcelRecord model already covers essential soil parameters such as volumetric water content (soilMoistureVwc), soil temperature (soilTemperature), and electrical conductivity (soilMoistureEC).
• However, there is no attribute to capture soil pH, which is critical for evaluating soil conditions and supporting sensor-driven agricultural applications.
• Adding this property ensures the model is compatible with multi-parameter soil sensors and aligns with common use cases in IoT-enabled agrifood systems.

Changes introduced:

• Added the soilPh property (type: number, description: Measured soil pH value, recommended scale 0–14) to schema.json and documentation.
• Included the property in example entities.

Please let me know if further clarification or documentation is needed!