Move man pages to man1 folder

This allows adding the repository's *man* directory directly to the
`MANPATH` environment variable, without installing. I find this useful
for my dotfiles repository; I can include git-extras as a submodule, so
anywhere I set up an environment, I can automatically have access to the
git-extras commands *and* manuals simply by cloning my dotfile repo
rather than having to run an installation command afterward.