[DOC] Add custom index management etc.

[kaituo]

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 and all versions that are affected.

• Starting 2.15, we support custom index management. For details, watch video in update create detector page and detector detail page to add custom result index lifecycle management settings anomaly-detection-dashboards-plugin#770
• Starting 2.15, we also make custom result index an alias. So we need extra security permission to manage it.
• Starting 2.15, we added a new setting plugins.anomaly_detection.jvm_heap_usage_threshold to manage memory circuit braeker threshold.
• We also add examples on DSL filter to improve documentation.

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