Add doc and shell completion generation commands

[bcl]

Use 'doc' to generate manpages, and 'completion' to generate shell completion scripts.

This changes to IMGREF in the usage string because manpage won't output

[ondrejbudai]

Thanks! Not sure if you know this, but we intend to ship this only as a container image, so having a man page might be a challenge, since containers generally lack the man tooling.

This makes me wonder how to properly do man pages (or offline documentation shipped with the tool itself in general) for a container: One of my ideas was to bundle README.md inside the image and teach the container to show it using a pager (like less(1)). Basically run podman run bootc-image-builder man and it would give you a paginated README.md. Alternatively, we can tie this somehow to what cobra generates.

I'm happy to hear your thoughts.

[bcl]

It might be useful to have them available even without man so that people exploring using (or extending) the container can generate the manpages and read them with cat :) Automated doc generation is always better than trying to keep a readme in sync.