Add Sphinx theme changelog and release notes

CHANGELOG.txt

@@ -4,6 +4,7 @@ Changelog
 2.13 (xx.xx.xxxx) - IN DEVELOPMENT
 ~~~~~~~~~~~~~~~~~
 
+ * The documentation now uses Sphinx Wagtail Theme https://github.com/wagtail/sphinx_wagtail_theme (Storm Heg, Tibor Leupold, Thibaud Colas and Coen van der Kamp).
  * New StreamField implementation with performance and functionality improvements (Matt Westcott, Karl Hobley)
  * Added a simple translation module into `wagtail.contrib.simple_translation` (Coen van der Kamp)
  * Add support for Django 3.2

docs/releases/2.13.rst

@@ -26,6 +26,13 @@ In this release, a new contrib app has been introduced called :doc:`simple_trans
 
 This module was contributed by Coen van der Kamp.
 
+Sphinx Wagtail Theme
+~~~~~~~~~~~~~~~~~~~~
+
+The `documentation <https://docs.wagtial.io/>` now uses `Sphinx Wagtail Theme <https://github.com/wagtail/sphinx_wagtail_theme>`.
+
+This theme is developed by Storm Heg, Tibor Leupold, Thibaud Colas and Coen van der Kamp.
+
 Other features
 ~~~~~~~~~~~~~~