docs: add index guidance to performance tuning

[evenyag]

What's Changed in this PR

Document schema and index design as performance-tuning levers.

• Add a query-tuning workflow that uses EXPLAIN ANALYZE VERBOSE and query predicates to identify index candidates.
• Explain index selection, costs, validation metrics, and the newly flushed data limitation.
• Link the performance guide to the table-design and data-index documentation.
• Align the table-design skill with how indexes are built for SST files produced by subsequent flushes.

Checklist

• Please confirm that all corresponding versions of the documents have been revised.
• Please ensure that the content in sidebars.ts matches the current document structure when you changed the document structure.
• This change requires follow-up update in localized docs.

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