User:Brion Vibber (WMF)/Thumbor docs and dev cleanup 2023

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Wikimedia's Thumbor plugin setup was recently retooled for modern Python 3 and should be much easier to work with in general, yay! But the code isn't super well documented and some of the Docker tooling's a little funky.


 * document external tool requirements
 * document external tool requirements
 * document external tool requirements
 * document external tool requirements