If it helps, a number of courses at my university used open source git based textbooks. For example, you can replicate this Statistics textbook using any static site generator designed for documentation: https://moderndive.com/ (https://github.com/moderndive/ModernDive_book/)

• https://rust-lang.github.io/mdBook/
• https://www.mkdocs.org/
• https://vitepress.dev/

We use vitepress for our docs: https://fedecan.ca/en/guide/get-started

Or you can get even simpler by using plain markdown files organized into folders.

In my opinion, it's better to start off simple while writing the content since then you can pick a tool based on what type of formatting you end up needing