docs: add sha256 checksums for 17.3.8

Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
(cherry picked from commit ba371c7262)

docs/relnotes/17.3.8.html

@@ -31,7 +31,8 @@ because compatibility contexts are not supported.
 
 <h2>SHA256 checksums</h2>
 <pre>
-TBD
+175d2ca9be2af3a8db6cd603986096d75da70f59699528d7b6675d542a305e23  mesa-17.3.8.tar.gz
+8f9d9bf281c48e4a8f5228816577263b4c655248dc7666e75034ab422951a6b1  mesa-17.3.8.tar.xz
 </pre>