Is there any decent GUI(Graphical User Interface) WYSIWYG(What You See is What You Get) editor (on any platform) that can be used for authoring/editing manpages? An app that can export to the troff format would also be fine.
There is an online editor for man pages in mdoc format: http://roperzh.github.io/grapse/
Mdoc is a semantic markup for man pages. It has been around for about two decades and supported by Linux and other Unix-like OSes. You should prefer mdoc to troff.