feat(kafka): add support for schema registries

[jschmid1]

Description

This PR adds comprehensive documentation about Schema Registry integration for Kafka-related plugins, including ASCII workflow diagrams to illustrate how Schema Registry works with each plugin.

Changes

• Added Schema Registry integration section to the Schema Registry overview page
• Added Schema Registry integration sections to all Kafka-related plugins:
  • Kafka Log
  • Kafka Upstream
  • Confluent
  • Kafka Consume
  • Confluent Consume
• Added ASCII workflow diagrams to visualize the Schema Registry integration process
• Added links to the Schema Registry configuration page from each plugin
• Added related resources sections with links to Confluent Schema Registry documentation

KAG: https://konghq.atlassian.net/browse/KAG-6636

Testing instructions

Preview link:

Checklist

• Review label added
• Conditional version tags added, if applicable.

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