toc: add TiDB release support policy (#19119) (#20860)

Author: ti-chi-bot

TOC.md

@@ -1092,6 +1092,7 @@
   - [All Releases](/releases/release-notes.md)
   - [Release Timeline](/releases/release-timeline.md)
   - [TiDB Versioning](/releases/versioning.md)
+  - [Release Support Policy](https://www.pingcap.com/tidb-release-support-policy/)
   - [TiDB Installation Packages](/binary-package.md)
   - v8.5
     - [8.5.1](/releases/release-8.5.1.md)