docs: optimise PNGs
I used zopflipng with 100 iterations on most of them, and advpng with 300 iterations in zopfli mode on front_page_screenshot.png
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Before Width: | Height: | Size: 19 KiB After Width: | Height: | Size: 18 KiB |
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Before Width: | Height: | Size: 11 KiB After Width: | Height: | Size: 11 KiB |
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Before Width: | Height: | Size: 7.6 KiB After Width: | Height: | Size: 7.2 KiB |
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Before Width: | Height: | Size: 5.4 KiB After Width: | Height: | Size: 5.1 KiB |
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Before Width: | Height: | Size: 9.2 KiB After Width: | Height: | Size: 8.9 KiB |