[DOC] Document RDS source plugin in Data Prepper

[oeyh]

What do you want to do?

• 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.
A new RDS source plugin is added to Data Prepper and needs corresponding documentation.

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
Data Prepper 2.11

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
I will work on this and open a PR for review.
Related Data Prepper issues:
opensearch-project/data-prepper#4561
opensearch-project/data-prepper#5309