docs: add sha256 checksums for 17.0.0

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

docs/relnotes/17.0.0.html

@@ -33,7 +33,8 @@ because compatibility contexts are not supported.
 
 <h2>SHA256 checksums</h2>
 <pre>
-TBD.
+696578f0b83796470511a88a95fff15a2a25fa201a9e487716f2ca20c177c3ab  mesa-17.0.0.tar.gz
+39db3d59700159add7f977307d12a7dfe016363e760ad82280ac4168ea668481  mesa-17.0.0.tar.xz
 </pre>