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

Got partway through a docs & dev cleanup of Thumbor, then got distracted. Getting back to this.

Thumbor update goals
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.

Want to:
 * make sure all tests run sensibly via Docker Desktop on Linux, Mac, and Windows hosts
 * make sure it's clearly documented and straightforward how to hook up a test instance to a MediaWiki-Docker instance to serve your thumbnails
 * improve documentation on how to add a file format to support
 * and in particular, make clear how to add test cases and how to document external tool requirements

Current todo:
 * submitted a partial cleanup a while ago to fix running tests on mac, check up on that and confirm all is well
 * clean up current offline state and resync
 * just churn through docs :D
 * make a clearer setup explainer for test usage