[DOC] [Multiple Datasource] Update documentation for AWS OpenSearch Serverlesss support in 2.8

[zhongnansu]

What do you want to do?
In upcoming 2.8 release, we plan to add support for AWS OpenSearch Serverlesss support, in multiple datasource. We need some updates to the documentation

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request and all versions that are affected.
https://opensearch.org/docs/latest/dashboards/discover/multi-data-sources/#creating-a-data-source-connection

Within the existing "aws sigV4" auth type configuration while creating a datasource, we will add an "service" option that user will choose between "Amazon OpenSearch Service" or "Amazon OpenSearch Serverless"

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

• original feature request issue: [Multiple Datasource] Support serverless AWS OpenSearch in SigV4 auth type OpenSearch-Dashboards#3623
• draft PR with video attached [Multiple Datasource] Support Amazon OpenSearch Serverless OpenSearch-Dashboards#3957

[ahopp]

Thanks for adding to v2.8 @hdhalter!