Fix images.

Currently they don't show up. They need to be within the `src/`
directory to get included in the generated output.

Author: nnethercote

guide/assets/nsight.png renamed to guide/src/assets/nsight.png

@@ -53,4 +53,4 @@ If you set up the codegen backend for debug, it should give you a mapping from R
 
 Here is an example of the screen you should see:
 
-![](../../../assets/nsight.png)
+![](../../assets/nsight.png)