[DOC] Add document for query insights health stats API

[ansjcy]

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.
We are introducing new APIs and metrics for getting health stats for query insights in 2.18, see more details in:

• [FEATURE] Enrich the Operational Metrics in Query Insights query-insights#9
• Add data models for health stats API query-insights#120
• Create health_stats API for query insights query-insights#122
• Otel counters for error metrics query-insights#124

We need to add related documentation for them as well.
*Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
2.18

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