docs: add sha256 checksums for 13.0.3

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit c8ece92ded9337b9ed60aa9568b41313025a1406)
diff --git a/docs/relnotes/13.0.3.html b/docs/relnotes/13.0.3.html
index 585dafa..59bc47d 100644
--- a/docs/relnotes/13.0.3.html
+++ b/docs/relnotes/13.0.3.html
@@ -31,7 +31,8 @@
 
 <h2>SHA256 checksums</h2>
 <pre>
-TBD
+55b07d056f9b855ba9d7c8b2ddc7d3b220a61c6ab1bdc73cbfc2f607721094c2  mesa-13.0.3.tar.gz
+d9aa8be5c176d00d0cd503cb2f64a5a403ea471ec819c022581414860d7ba40e  mesa-13.0.3.tar.xz
 </pre>