[AWS::DocDB::DBInstance] - [Coverage] - Add support for PerformanceInsightsKMSKeyId property

### Name of the resource

AWS::DocDB::DBInstance

### Resource name

_No response_

### Description

The [AWS::DocDB::DBInstance](https://docs.aws.amazon.com/AWSCloudFormation/latest/TemplateReference/aws-resource-docdb-dbinstance.html) resource type in CloudFormation does not support "PerformanceInsightsKMSKeyId" property (customer managed KMS key for Performance Insights).

Whereas the Document DB console, CreateDBInstance API as well as [Terraform](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/docdb_cluster_instance.html) supports it.

### Other Details

_No response_

Start with the AWS::DocDB::DBInstance resource documentation and the CreateDBInstance API entry point, then compare the existing CloudFormation property coverage with the documented PerformanceInsightsKMSKeyId behavior. Done means the resource supports the customer-managed KMS key property consistently with the DocumentDB API and console.

Written by the indexing model from the issue text.