Add new Index advisor user doc (#20011)

Author: Oreoxmt

TOC.md

@@ -369,6 +369,7 @@
       - [执行计划管理](/sql-plan-management.md)
       - [优化规则及表达式下推的黑名单](/blocklist-control-plan.md)
       - [Optimizer Fix Controls](/optimizer-fix-controls.md)
+    - [索引推荐 (Index Advisor)](/index-advisor.md)
 - 教程
   - [单区域多 AZ 部署](/multi-data-centers-in-one-city-deployment.md)
   - [双区域多 AZ 部署](/three-data-centers-in-two-cities-deployment.md)