Updated Paint Data widget documentation

[borondics]

Issue

Description of changes

Added new screenshots and modified the text accordingly.

I'm not a fan of the example though. It talks more about the clustering than Paint Data itself and I think it is not easy to see what the difference between k-means and hierarchical clustering is in this case with this layout. In any case, I'll leave it as is for now, with an updated image.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.47%. Comparing base (0ce81e5) to head (fbef94d).

Additional details and impacted files

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##           master    #6977   +/-   ##
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  Coverage   88.47%   88.47%           
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  Files         332      332           
  Lines       73287    73287           
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  Hits        64841    64841           
  Misses       8446     8446           

[markotoplak]

The images are too big (in file size, as in KB) for what they are for. They are made in retina resolution, which is probably unnecessary for the docs. We try to keep the cumulative help size as small as possible.

Also, keep widgets as small as possible while still showing the interface nicely. This allows users to use smaller help windows.

[markotoplak]

@borondics, we discussed this in the lab. So we will allow retina images, BUT we won't allow any image resizing. Therefore, screenshots should be added in exactly the same size as they were taken, and any necessary resizing should go into the .md file (where we have to somehow specify that retina images are shown in 50% scale).