AWS::DocDB::Index

### Name of the resource

AWS::DocDB::DBCluster

### Resource name

_No response_

### Description

It could, under certain circumstances, be very helpful if DocumentDB collection indexes could be made via a CloudFormation resource. Would love to hear thoughts from other users too on this, in case I may have missed an obvious lifecycle-related best opportunity for index creation. But CloudFormation resources to me seem like they would make creation/deletion very seamless.

### Other Details

I was thinking something along these lines:

```yaml
MyCollectionIndex:
Type: AWS::DocDB::Index
Properties:
SecretId: !Ref MyDocumentDbCredentials
CollectionName: myCollection
Name: myIndex
Keys:
- Field: myField
Order: asc # either "asc" or "desc"
Unique: false # either true or false (optional, default: false)
Sparse: false # either true or false (optional, default: false)
```