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)
Isn't just `go get` sufficient?  From `go help get`:
```console
$ go help get
usage: go get [-d] [-fix] [-t] [-u] [build flags] [packages]

Get downloads and installs the packages named by the import paths,
along with their dependencies.

...
```