[2.2.x] Added stub release notes for 2.2.25.

Backport of ae4077e from main.

Author: felixxm

docs/releases/2.2.25.txt

@@ -0,0 +1,9 @@
+===========================
+Django 2.2.25 release notes
+===========================
+
+*December 7, 2021*
+
+Django 2.2.25 fixes a security issue with severity "low" in 2.2.24.
+
+...

docs/releases/index.txt

@@ -25,6 +25,7 @@ versions of the documentation contain the release notes for any later releases.
 .. toctree::
    :maxdepth: 1
 
+   2.2.25
    2.2.24
    2.2.23
    2.2.22