Switch sphinx man_pages generation to use commandline/cli

... instead of toctree for a more relevant/useful man page.

This fixes #1957 and #1958.

Right now, if we run `make -C docs man`, we get a big monolithic man page that contains _all_ the documentation we've got.  This switches it to just be a copy of the main reference for the CLI, which is fairly lightweight (especially in comparison), and matches more closely what users would expect from typing `man docker`.