Use pure go markdown processor to generate man files

Fixes #7319

Docker-DCO-1.1-Signed-off-by: Brian Goff <cpuguy83@gmail.com> (github: cpuguy83)
~~~
go-md2man > `go-md2man` (I think! - ping @jamtur01)
~~~

go > Go

markdown > Markdown

I am not sure if "manfile" is a thing? (as in, a noun. perhaps all "manfile(s)" should be replaced with "man page(s)"?)