docs: add sha256 checksums for 12.0.0

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 3a146a789c5aa45e0de52bff764c2b371dd253aa)
diff --git a/docs/relnotes/12.0.0.html b/docs/relnotes/12.0.0.html
index 15958fd..4f11bbd 100644
--- a/docs/relnotes/12.0.0.html
+++ b/docs/relnotes/12.0.0.html
@@ -33,7 +33,8 @@
 
 <h2>SHA256 checksums</h2>
 <pre>
-TBD.
+3b8fa4d86d78f8f6ec86055b92ad1afe869001483593b3dd4531184b8bc4fcfb  mesa-12.0.0.tar.gz
+0090c025219318935124292b482e3439bc43e8c074ad01086449fcad88547dc6  mesa-12.0.0.tar.xz
 </pre>