v7.5.7: add release notes (#21463)

Author: Oreoxmt

TOC.md

@@ -1118,6 +1118,7 @@
   - v7.6
     - [7.6.0-DMR](/releases/release-7.6.0.md)
   - v7.5
+    - [7.5.7](/releases/release-7.5.7.md)
     - [7.5.6](/releases/release-7.5.6.md)
     - [7.5.5](/releases/release-7.5.5.md)
     - [7.5.4](/releases/release-7.5.4.md)