releases: add release notes for v8.4.0 (#18556) (#19036)

README.md

@@ -15,6 +15,7 @@
 | 文档仓库 branch | 对应 TiDB 文档版本 |
 |:---------|:----------|
 | [`master`](https://github.com/pingcap/docs-cn/tree/master) | dev 最新开发版 |
+| [`release-8.4`](https://github.com/pingcap/docs-cn/tree/release-8.4) | 8.4 开发里程碑版 (DMR) |
 | [`release-8.3`](https://github.com/pingcap/docs-cn/tree/release-8.3) | 8.3 开发里程碑版 (DMR) |
 | [`release-8.2`](https://github.com/pingcap/docs-cn/tree/release-8.2) | 8.2 开发里程碑版 (DMR) |
 | [`release-8.1`](https://github.com/pingcap/docs-cn/tree/release-8.1) | 8.1 长期支持版 (LTS) |

TOC.md

@@ -4,7 +4,7 @@
 - [文档中心](https://docs.pingcap.com/zh)
 - 关于 TiDB
   - [TiDB 简介](/overview.md)
-  - [TiDB 8.3 Release Notes](/releases/release-8.3.0.md)
+  - [TiDB 8.4 Release Notes](/releases/release-8.4.0.md)
   - [功能概览](/basic-features.md)
   - [与 MySQL 的兼容性](/mysql-compatibility.md)
   - [使用限制](/tidb-limitations.md)
@@ -1077,6 +1077,8 @@
   - [版本发布时间线](/releases/release-timeline.md)
   - [TiDB 版本规则](/releases/versioning.md)
   - [TiDB 离线包](/binary-package.md)
+  - v8.4
+    - [8.4.0-DMR](/releases/release-8.4.0.md)
   - v8.3
     - [8.3.0-DMR](/releases/release-8.3.0.md)
   - v8.2