docs: make subcommands organization example be more explicit

[lvjp]

It's always difficult to be sure that we understand the subcommands organization if we cannot match the example.
In the real world we are facing a lot of time leaf and subcommand at the same level. Just put it into the example.

If we take the well-known Docker command (simplified) as an example:

├── ps
└── container
    ├── kill
    ├── ls
    └── logs

Should I put ps into cmd/ps.go beside root.go or into cmd/ps/ps.go ?

I think that the documentation is for having cmd/ps.go :

The suggested approach is for the parent command to use AddCommand to add its most immediate subcommands.

But an example is better than words!

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