[DOC] Flatten Custom Result Index in Anomaly Detection

[jackiehanyang]

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.
related github issue - opensearch-project/anomaly-detection#1306

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

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
related pr - opensearch-project/anomaly-detection#1401

[Naarcha-AWS]

Relates to PR #9140