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)
Oh gorgeous.  Can we get this `v1` tag cloning pretty in the main Dockerfile too?  (preferably all on one line there with `go get -v` - here I don't care much :P)

ie:
```dockerfile
# Install manfile generator
RUN mkdir -p /go/src/github.com/cpuguy83 \
    && git clone -b v1 https://github.com/cpuguy83/go-md2man.git /go/src/github.com/cpuguy83/go-md2man \
    && cd /go/src/github.com/cpuguy83/go-md2man \
    && go get -v ./...
```