CosmosDB: Guidance on extracting partitionkey from messages should be on separate page

[ramonsmits]

Currently, the page that explains how to extract the partition key from messages is documented at https://docs.particular.net/persistence/cosmosdb/transactions where the sub menu item is called "Transactions".

It is better to have separate pages that explains how transactions/consistency works with NServiceBus.Persistence.CosmosDB and how the partition key can be extracted.