[Resource Type] - [Docs] - Improve documentation clarity for OpenSearchServerless Index Mappings Properties notation

### Name of the resource

Other

### Resource name

AWS::OpenSearchServerless::Index

### Reference Link

https://docs.aws.amazon.com/AWSCloudFormation/latest/TemplateReference/aws-properties-opensearchserverless-index-mappings.html

### Details

## Issue Description
The current documentation for AWS::OpenSearchServerless::Index Mappings Properties is unclear about the required key-value pair notation structure. This makes it difficult for users to understand how to properly format their CloudFormation templates.

## Current Documentation
The current documentation at [AWS::OpenSearchServerless::Index Mappings](https://docs.aws.amazon.com/AWSCloudFormation/latest/TemplateReference/aws-properties-opensearchserverless-index-mappings.html) doesn't provide clear examples of the required structure.

## Example of Invalid Template
```yaml
Mappings:
Properties:
Dimension: 1536
Type: knn_vector
```
## Example of Valid Template
```yaml
Mappings:
Properties:
test: # Required field name
Dimension: 1536
Type: knn_vector
```

## Suggested Improvements
- Add complete working examples to the documentation
- Clarify that a field name is required as a key in the Properties object
- Include more detailed explanation of the structure and its requirements
- Add common use cases and their corresponding template structures
## Impact
### This improvement would help users:
- Avoid trial and error when creating templates
- Better understand the required structure
- Save time in template development
- Reduce support queries related to this issue
-
Would you please consider updating the documentation to make it more clear and user-friendly?

Start by reviewing the AWS::OpenSearchServerless::Index Mappings reference linked in the issue and compare its current notation with the invalid and valid YAML examples provided. Update the documentation with a clear field-name key explanation and complete working examples; done means users can understand and reproduce the required Properties structure.

Written by the indexing model from the issue text.