This can be used as a Rust library or as a command-line executable.
Install Rust 1.42 or later
Run cargo install mandown
The mandown command takes a path as an argument, and prints manpage to stdout.
sh
mandown README.md > converted.1
man ./converted.1
You can specify path as - to read markdown from stdin. Second and third argument can specify program name and manpage section.
sh
cat README.md | mandown - MYPROGRAM 1 > converted.1
man ./converted.1