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You know what, while you're at it, I might as well bring it up. How about XML? I haven't really tried writing large pieces of text in it but I am already seriously considering. All other alternatives are too complicated and have a learning curve that gets in the way of writing itself. With XML, I'd be able to define my own tags and run them by a parser later on to auto-generate indexable footnotes, and create my own…
Professional tech writer here: We use GitHub and a tool called OxygenXML to write docs-as-code in an XML DTD called "DITA". It's a hefty IBM invention from the early aughts, but it covers every use case I've thrown at it, from small documentation sets to multi-thousand-page monsters. Supports PDF, HTML, Word, and many other output types.
DITA's benefits require a certain scale that most tech companies never achieve. And the Open Toolkit is a nightmare piece of software.