[IOT Lab] Suggestion: More clear mapping of partition key choice to the 3 collections

[deborahc]

https://github.com/AzureCosmosDB/scenario-based-labs/blob/master/IoT/HOL%20step-by%20step%20-%20Cosmos%20DB%20scenario-based%20labs%20-%20IoT.md#about-cosmos-db-partitioning

This section explains why various partition keys were chosen. It describes using vin for the partition key value for vehicle and trip metadata and consignment id for the package metadata. It's not immediately clear at this stage in the lab which collection this is for.

Could we go through each collection systematically and explain why the partition key was chosen?