add benchmark doc to toc (#21820)

Author: hfxsd

TOC.md

@@ -1084,6 +1084,7 @@
   - [Schedule Replicas by Topology Labels](/schedule-replicas-by-topology-labels.md)
   - [URI Formats of External Storage Services](/external-storage-uri.md)
   - [TiDB Workload Repository](/workload-repository.md)
+  - [Interaction Test on Online Workloads and `ADD INDEX` Operations](/benchmark/online-workloads-and-add-index-operations.md)
 - FAQs
   - [FAQ Summary](/faq/faq-overview.md)
   - [TiDB FAQs](/faq/tidb-faq.md)