docs: Add sha256 sums for the 10.5.1 release

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

docs/relnotes/10.5.1.html

@@ -31,7 +31,8 @@ because compatibility contexts are not supported.
 
 <h2>SHA256 checksums</h2>
 <pre>
-TBD
+b5b6256a6d46023e16a675257fd11a0f94d7b3e60a76cf112952da3d0fef8e9b  mesa-10.5.1.tar.gz
+ffc51943d15c6812ee7611d053d8980a683fbd6a4986cff567b12cc66637d679  mesa-10.5.1.tar.xz
 </pre>