[Data Prepper] Correct Kafka source documentation

[Naarcha-AWS]

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.

Address the following user feedback for the Kafka source page

• The Insecure option under ENCRYPTION starts with uppercase "I", causing runtime exceptions
• Incorrect descriptions for registry_url and version fields in Schema block
• Missing SSL authentication information for Schema (users report having to search GitHub for this)
• General lack of information about SSL certificates for Kafka
• Documentation needs more examples and comprehensive information about configuration options

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

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

[natebower]

@dlvenable Is this something you or your team could address?