releases: add release notes for v8.4.0 (pingcap#18829)

Author: Oreoxmt

README.md

@@ -28,6 +28,7 @@ Currently, we maintain the following versions of TiDB documentation in different
 | Branch name | TiDB docs version |
 | :---------|:----------|
 | [`master`](https://github.com/pingcap/docs/tree/master) | The latest development version |
+| [`release-8.4`](https://github.com/pingcap/docs/tree/release-8.4) | 8.4 Development Milestone Release |
 | [`release-8.3`](https://github.com/pingcap/docs/tree/release-8.3) | 8.3 Development Milestone Release |
 | [`release-8.2`](https://github.com/pingcap/docs/tree/release-8.2) | 8.2 Development Milestone Release |
 | [`release-8.1`](https://github.com/pingcap/docs/tree/release-8.1) | 8.1 LTS (Long-Term Support) |

TOC.md

@@ -4,7 +4,7 @@
 - [Docs Home](https://docs.pingcap.com/)
 - About TiDB Self-Managed
   - [What is TiDB Self-Managed](/overview.md)
-  - [TiDB 8.3 Release Notes](/releases/release-8.3.0.md)
+  - [TiDB 8.4 Release Notes](/releases/release-8.4.0.md)
   - [Features](/basic-features.md)
   - [MySQL Compatibility](/mysql-compatibility.md)
   - [TiDB Limitations](/tidb-limitations.md)
@@ -1073,6 +1073,8 @@
   - [Release Timeline](/releases/release-timeline.md)
   - [TiDB Versioning](/releases/versioning.md)
   - [TiDB Installation Packages](/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