What am I missing here? Hasn't SoftIron been building this exact thing for around 5 years? Heck, they design and manufacture all of the hardware as well.
I'd recommend AsciiDoctor. It provides a syntax that's very markdown-like. Store your book as src in Git somewhere and move seamlessly between Fedora and your Mac. There's a nice VSCode AsciiDoctor plugin and an extremely helpful community. And it outputs PDF, DocBook, epub, and HTML.
I really like asciidocs support for diagramming via the asciidoctor-diagram extension. It's supports lots of unique and useful diagramming formats, such as graphviz, ditaa, bytefield, etc.
Intel® Pathfinder for RISC-V supports Bare Metal, FreeRTOS, Zephyr OS and Yocto Linux.
Intel Pathfinder for RISC-V is scalable from individual users in academia and research, all the way to large-scale commercial projects. The table below summarizes the feature sets available currently, with new and exciting features being added on an ongoing basis.
“I don’t believe anything really revolutionary has ever been invented by a committee… i’m going to give you some advice that might be hard to take. That advice is: Work alone… not on the committee. Not on a team” — Steve Wozniak
Don't know the details of Google's setup, but they're creating a screencast with a tablet. There are several ways to do that depending on your hardware, but if you search for 'creating screencast with tablet' you'll find several examples.