If Ciro Santilli were to write a book about quantum mechanics as of 2020 (before OurBigBook.com went live), he would upload an OurBigBook Markup website to GitHub Pages.
But there is one major problem with that: the entry barrier for new contributors is very large.
If they submit a pull request, Ciro has to review it, otherwise, no one will ever see it.
Our amazing website would allow the reader to add his own example of, say, The uncertainty principle, whenever they wants, under the appropriate section.
Then, people who want to learn more about it, would click on the "defined tag" by the article, and our amazing analytics would point them to the best such articles.