I use asciidoc to write documentation for this.
I think it is nicer than Markdown. Sadly, A few things don’t work entirely when rendered by GitHub.
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It seems to run in 'SECURE' mode, so includes are not working
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It seems to also miss things like support
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for LaTeX formulas
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embedding of YouTube videos.
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adding docinfo, which is used here and there to add javascript and style for working demo’s
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I worked around the first thing by a GitHub action 'reducing' the .adoc, which effectively resolves the includes (and produces the README.adoc files from the README-source.adoc files). This way some modules have their own documentation, which is then included as chapter of the overall documentation. Also, includes are used to include example code from actual code (like test cases). The README.adoc files then render directly 'mostly' as intended.
The other things (formula’s, video’s and docinfo) could (I presume) only be worked around by transforming the entire thing to HTML, which happens in the publish-documentation action. A drawback is that the documentation is less tight to GitHub itself, and you need to access a different url, but every detail should be working there.