feat(init): create install-man-pages subcommand

[arxanas]

Closes #828.

[alerque]

I think this looks fine. As I understand it there will be a new dedicated subcommand to generate man pages, with an optional argument for where to place the generated file(s). That should work for distro packaging purposes.

A couple comments I wasn't clear on from skimming the PR:

• Will the existing init subcommand still try to install man pages locally? I would suggest it not do that by default.
• Will the man page install subcommand be hidden from --help? I typically suggest it my a hidden subcommand, but document it in the project readme or install instructions so distro folks in particular catch it but it doesn't clutter up usage info for end users that may not have access to the root system anyway and shouldn't be stuffing stuff there behind their package manager's back unless they know what they are doing.

[arxanas]

Will the existing init subcommand still try to install man pages locally? I would suggest it not do that by default.

No, it currently doesn't install man pages at all (and it still won't).

Will the man page install subcommand be hidden from --help? I typically suggest it my a hidden subcommand, but document it in the project readme or install instructions so distro folks in particular catch it but it doesn't clutter up usage info for end users that may not have access to the root system anyway and shouldn't be stuffing stuff there behind their package manager's back unless they know what they are doing.

The majority of git-branchless users install from source and won't have any way to install man pages (which breaks git <command> -h, see #88 (comment)), so I think I'll leave it as visible in case someone does want to install man pages on their own (at their own risk).