Document the internal workings of the VisualEditor source code on MediaWiki.org, at an appropriate level of detail for onboarding an engineer unfamiliar with VisualEditor, providing pointers and explanations so they can self-familiarize with the source code.
Entry points:
https://www.mediawiki.org/wiki/VisualEditor/Internals
https://www.mediawiki.org/wiki/VisualEditor/Internals/DM
https://www.mediawiki.org/wiki/VisualEditor/Internals/CE
https://www.mediawiki.org/wiki/VisualEditor/Internals/UI
Existing pages that are also relevant (but may need updating):
https://www.mediawiki.org/wiki/VisualEditor/Developers/Getting_started
https://www.mediawiki.org/wiki/VisualEditor/Design/Software_overview
https://www.mediawiki.org/wiki/VisualEditor/Design#Historical_pages_(not_current)
https://www.mediawiki.org/wiki/VisualEditor/Presentations