docs: add sha256 checksums for 13.0.0

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 405dd26860719d800ed6134f8f985f1525f25502)
diff --git a/docs/relnotes/13.0.0.html b/docs/relnotes/13.0.0.html
index a3b3240..8258066 100644
--- a/docs/relnotes/13.0.0.html
+++ b/docs/relnotes/13.0.0.html
@@ -33,7 +33,8 @@
 
 <h2>SHA256 checksums</h2>
 <pre>
-TBD.
+4a54d7cdc1a94a8dae05a75ccff48356406d51b0d6a64cbdc641c266e3e008eb  mesa-13.0.0.tar.gz
+94edb4ebff82066a68be79d9c2627f15995e1fe10f67ab3fc63deb842027d727  mesa-13.0.0.tar.xz
 </pre>