docs: add sha256 checksums for 12.0.3

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 09460b8cf7ddac4abb46eb6439314b29954c76a6)
diff --git a/docs/relnotes/12.0.3.html b/docs/relnotes/12.0.3.html
index cc59a01..70e704b 100644
--- a/docs/relnotes/12.0.3.html
+++ b/docs/relnotes/12.0.3.html
@@ -31,7 +31,8 @@
 
 <h2>SHA256 checksums</h2>
 <pre>
-TBD
+79abcfab3de30dbd416d1582a3cf6b1be308466231488775f1b7bb43be353602 mesa-12.0.3.tar.gz
+1dc86dd9b51272eee1fad3df65e18cda2e556ef1bc0b6e07cd750b9757f493b1 mesa-12.0.3.tar.xz
 </pre>