Documentation on how to implement `--version` flag

[smheidrich]

Is your feature request related to a problem?

It's very common (almost mandatory) for CLI tools to provide a --version option which, when used, will just print the tool's version and exit. I couldn't find anything in argh's docs about how to best/most idiomatically implement this in (modern) argh.

Describe the solution you'd like

Somewhere in the docs there should be a short paragraph/example on how to implement best implement --version.

This might be a good candidate for the cookbook section of the docs if it's not a built-in feature (see last secion of this issue) and requires a bit of boilerplate.

Describe alternatives you've considered

• Don't document it and let people come up with their own solutions.
• Only document it once additional features that make it easier to implement have been added to argh (which ones?).

Additional context

argparse makes implementing this as easy¹ as adding an argument with action="version". AFAIU, argh uses argparse under the hood and exposes it where possible, so maybe this can be leveraged?

^{1 Compare this to how it's how it's done in Typer... Although to be fair, Typer seeks to provide a generic mechanism instead of special-casing it like argparse does.}