docs: Add sha256 sum for 23.0.3

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22635>
diff --git a/docs/relnotes/23.0.3.rst b/docs/relnotes/23.0.3.rst
index 270d046..4d6c350 100644
--- a/docs/relnotes/23.0.3.rst
+++ b/docs/relnotes/23.0.3.rst
@@ -19,7 +19,8 @@
 
 ::
 
-    TBD.
+    386362a5d80df3b096636b67f340e1ce67b705b44767d5bdd11d2ed1037192d5  mesa-23.0.3.tar.xz
+
 
 
 New features