A quick vote for commonmark:
- it's very easy to read and write - even in as raw text;
- it's well supported by GitHub;
- it does 90% of what you need to write technical documentation;
- it allows you to casually drop snippets of HTML/JSON in for the other 10% and
- there are plenty of tools for converting it to HTML and other formats (see Pandoc and cmark).
In fact the above rant is commonmark! https://gist.github.com/johnmcfarlane/aec9d1c55d25bbbc086469c0c897b580