Add comprehensive CLI tool docs

[lucperkins]

At the moment, various CLI tools are mentioned in passing but not really documented. We should decide which tools require documentation and provide a simple YAML-based mechanism for generating those docs.

[mikebrow]

I'd start with containerd (the daemon) and ctr (the containerd cli)
containerd: https://github.com/containerd/containerd/blob/master/docs/man/containerd.1.md
ctr: https://github.com/containerd/containerd/blob/master/docs/man/ctr.1.md

crictl also needs documenting but it's just for the kubernetes/cri use case. We have a crictl doc here: https://github.com/containerd/cri/blob/master/docs/crictl.md

[mikebrow]

It's easy to generate toml from the configuration options as we already have something for that, but the command line options are all written up in golang to create urfave command line options and I don't know of a tool to generate yaml/md/toml from urfave yet. So we would probably have to refactor the code to get what you are asking for here. Or do a pr for this issue: urfave/cli#340

[lucperkins]

@mikebrow If memory serves, I think that @estesp would prefer not to include ctr documentation in the https://containerd.io docs. Phil, let me know if I'm mistaken in that! Happy to include whatever.

As for urfave, it definitely would be nice to be able to generate YAML from that directly, but in the meantime I think that hand-written YAML could be a good medium-term solution.

[mikebrow]

The issue says:

various CLI tools

As for:

@mikebrow If memory serves, I think that @estesp would prefer not to include ctr documentation in the https://containerd.io docs. Phil, let me know if I'm mistaken in that! Happy to include whatever.

that's a tough call..

ctr is used, may be necessary in certain debug/support situations and it is difficult to use it. Higher level tools that include containerd arguably should be documented elsewhere. Thus begging the question, if not ctr, which CLI tools should be documented here :-)