[nim-markdown](https://github.com/soasme/nim-markdown) might qualify for your
reuqirements. It has a [dev
guide](https://enqueuezero.com/markdown-parser.html) that gives you a simple
example on how to write a parser. Also it is pretty flexible in the sense that
you might be able to customize Nim markdown itself for your scope. A markdown
parser is configure with a number of block parsers and online parser and you
might need only to customize those, see for example this definition of GitHub
flavored markdown parser:
<https://github.com/soasme/nim-markdown/blob/e6379e25a33d1d79bb62ee285574e41a395ae2d7/src/markdown.nim#L2457>
