I couldn't find a library for this or simple guide to this job (note: especially for manual pages).
I'm not familiar with ROFF, but I assume the roff crate isn't working for you?
Probably a random side note, but Cargo uses the mdman rustc crate. According to Format, it supports plain text as output format, but unfortunately I don't think it supports ROFF as input (only markdown + handlebars templates).
In this case, I think I have to make my own library. I doubt about roff markdown sections every time when I try to develop a man alternative in Rust, even with documentation (also it's bit hard to find them because they seem like old documents)