docs: explain KEDA automatic scaler discovery

[aryanbarde80]

Problem

The KEDA trait reference documents the keda.auto option but does not explain how automatic discovery works, what a KEDA scaler is, or which Camel components currently have automatic scaler mappings. This makes it difficult for users to understand what auto will and will not configure.

Solution

• Add an “Automatic scaler discovery” section to the KEDA trait reference.
• Explain how Camel K derives KEDA triggers from consumer endpoint URIs.
• Document the currently supported automatic mapping: kafka.
• Link to the KEDA scaler concepts and Apache Kafka scaler documentation.
• Clarify that unsupported components can still use explicit keda.triggers configuration.

The new section is outside the autogenerated configuration block, so no generated content was manually modified.

Testing

• git diff --check — passed.
• make update-docs — attempted, but could not complete because the contributor environment does not have the required go and yq executables.
• The generated page was restored afterward, and the final diff was verified to contain only the intended documentation change.

Fixes #6891