Learning LaTex and tiKz help out with this. It looks like a presentation. So latex beamer package with some custom templates. The downside is that latex and tikz has a little bit of a learning curve. But it is worth it in the long run.

Have you used Katex in place of Latex? The latter is much lighter, especially if you're trying to publish a tome for the web.

No, I haven't. But I'll check it out! I had to go the full route a while back. Yes, it was bulky, but got the job done.

That's part of it, but the content still flows top-to-bottom for the most part and is not quite enterprise-ready, packaged into a standalone app.