docs: Update docs for the aws_kendra_index resource

[makotonakai]

Description

This PR adds GEN_AI_ENTERPRISE_EDITION edition to the aws_kendra_index resource

Relations

Closes #41390

References

The announcement for GenAI index
https://aws.amazon.com/about-aws/whats-new/2024/12/genai-index-amazon-kendra/

The source of the new edition
https://github.com/aws/aws-sdk-go-v2/blob/main/service/kendra/types/enums.go#L676

Output from Acceptance Testing

Not required, because this PR only updates the documentation.

[github-actions]

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