Document CosmosDB Partitioned Queries

[berndverst]

In CosmosDB State store Query API (which is Alpha) we only support cross-partition queries. However, the ability to query a specific partition (which is cheaper) has been added now:
dapr/components-contrib#3227

This needs to be documented here:
https://github.com/dapr/docs/blob/v1.12/daprdocs/content/en/reference/components-reference/supported-state-stores/setup-azure-cosmosdb.md

It would be great to introduce a
Query API (Alpha) section there that goes into a specific request example.

Additionally the Partition Key section could be improved also.

Hoping @luigirende will write this docs update :)

[luigirende]

/assign