Remove colorcube-analyze documentation

The 'colorcube-analyze' tool was also used as a visualization method in the 'posterize' documentation, which has been replaced now. The replacement does not show any histogram, since I don't think it adds relevant information. As a side effect, the 'posterize' documentation was slightly restructured to follow the filter template (although it is not listed as a filter, but a menu item)

I didn't have the original sunflower image, so I went with another one from unsplash.com.