AsciiDoc is really underrated. It's the only format that is simple and readable like MarkDown and yet is powerful enough to write whole books.

That may be because of the vicious cycle of lack of popularity, which leads to worse tool support, which leads to people preferring Markdown-based tools, which leads to increased spread of markdown and less popularity for AsciiDoc.

One lesser known fact of AsciiDoc is that it's a markup syntax for DocBook, so it supports the full feature set of this book-creating toolchain. Markdown can only dream of having support for more than simple web pages and single-page documents.

AsciiDoc is also supported by GitHub, I just use README.adoc instead of README.md.

The IntelliJ plugin is excellent.

And https://asciidoctor.org/docs/asciidoctor-diagram/ is amazing. I use PlantUML in it and love it.