Is anyone aware of a self hosted headless CMS that can output markdown?

I've never seen Markdown as the output of a CMS (e.g. let nontechnical users make arbitrary edits, then simplify the format to Markdown). If at all I've seen Markdown as input for systems. I.e. Liferay's knowledgebase can take Markdown input.
On the hard criteria of "outputting" Markdown, I can only think of git, which is contrary to your "nontechnical users".