Exercise solution should be commented and understandable

[keryell]

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We got a problem for example with https://github.com/codeplaysoftware/syclacademy/blob/main/Code_Exercises/Exercise_15_Image_Convolution/reference.cpp when everybody were trying to understand the details, including the presenter. @AerialMantis please add comments. :-)

[AerialMantis]

Thanks for pointing this out, usually I present this one, I can see this being difficult to follow so I'll update it with comments.