Add a Plastic image compare screenshot

Dependencies

• [2] 36IPNPIS Handle a few TODOs
• [*] UH3YXOLF Start the slide deck
• [*] 4AZS4L6B Add chapters
• [*] 5LKTNB6O LOTS of progress

Change contents

• file addition: plastic-image-compare.png (----------)
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• edit in sections/4-four-new-kids-on-the-block.md at line 56
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  + ---

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  + <!-- .slide: data-background="img/plastic-image-compare.png" -->
• edit in sections/4-four-new-kids-on-the-block.md at line 61
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  + <https://www.plasticscm.com/images/gallery/large/image-02-diff-swipe.png> <!-- .element: class="attribution" -->

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