images generator for documentation page

[dkazanc]

After #134 is implemented. Generate an image, as results of the method being applied to synthetic data. Arguably, this shouldn't be a part of the documentation building process as it would require httomolibgpu dependency and also GPU availability. I suggest we have a script that generates an image for each method and then the image is picked up during the documentation build.

Testing: In theory, the test can also generate an image and then that image is compared with the one that is available already. Could be useful to check if a) the image is missing, b) the image changed, meaning that the result of the method has changed.