add `help` command for consistent UX

[Kangie]

If nattka uses commands like nattka apply, nattka commit, etc. it only makes sense to support nattka help as a command as well - to do otherwise is inconsistent with the rest of the nattka UX.

Additionally, nattka help command should show the appropriate help, perhaps with an example?

$ nattka help make-package-list
nattka make-package-list attempts to create a package list for keywording PACKAGE (along with any dependencies).

usage:
    nattka make-package-list [-h] [-a ARCH] [-p PROFILES] [-s] package [package ...]

examples: 
    $ nattka make-package-list -s 'www-client/chromium-116.0.5845.140' -a amd64 -a arm64

positional arguments:
. . .

[arthurzam]

I think in python world, the normal interface is command subcommand --help. This is also the interface for git as example, so isn't only python world.

[thesamesam]

See https://clig.dev/#help which agrees with the original issue, although funnily enough it says "if you're git-like, you should support help", even though git doesn't...