I'd be surprised if it's not easy to transpile a Markdown document into the format

By hand—if you have experience writing roff typesetting—it is.

Having a program do it for you... you're going to get something, but it won't be correct and you will need to fix most of it.

A few problems come to mind:

1. It's a macro-based typesetting language. As a consequence, there's a one-to-many association between representations in Markdown with some equivalent in roff. A Markdown paragraph is just a paragraph, but in roff it could be an un-indented paragraph, a paragraph with first-line indentation, a paragraph with line-wrap indentation, or a paragraph with a left margin.

2. Rendering a man page, you have multiple different implementations of man and multiple different implementations of *roff (roff, troff, groff, nroff). The set of macros and features that are available differ depending on which implementation, resulting in one-size-fits-all solutions targeting the lowest common denominator.

3. Ironically, the one-to-many association goes both ways. With Markdown, you have code fences, quotes, italic text, bold text, and tables. With lowest-common-denominator manpage roff, you have paragraphs and emphasis that will either be shown as bold or inverted. If you're lucky, you might also be able to use underlines. If Markdown tables are no wider than 80 characters, you could preprocess those into plain characters, at least.

4. Despite being more structured with its typesetting, the contents of a manpage are mostly still unstructured. The individual sections within the page and its use of indentation and emphasis are entirely convention, and not represented in the source code by anything more than just typesetting macro primitives.

It could work out if you generate both the Markdown and man page from something with more explicit structure. If the plan is to go from a loose Markdown document into a manpage, you're going to end up having to write your Markdown document almost exactly like a manpage.