[DOC] Data Prepper secrets extension documentation

[chenqi0805]

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 and all versions that are affected.
The pipeline_configurations:aws:secrets will be a Data Prepper extension plugin shared across pipeline plugins (source/processor/sink) and will be released in Data Prepper 2.5. This extension plugin will read secrets from AWS secret manager.

I will create a PR for this

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