Provide examples to each major in-toto cli subcommand

[shibumi]

According to the Cobra documentation, Cobra supports examples. I think we should use these example strings for making in-toto easier to use. This might be something for @alanssitis. Do you want to have a look? :)

[alanssitis]

@shibumi could you link the documentation you are referring to? I do not understand what you mean by example strings.

[developer-guy]

hello @alanssitis, IMHO, @shibumi refers to the command's long description, you can use this field to add examples of using this command. For example, you can look at the example of the completion command.

👀 https://github.com/in-toto/in-toto-golang/blob/master/cmd/completion.go#L18

[alanssitis]

Yeah @shibumi , I can definitely take a look into it! Thank you @developer-guy for pointing me to the right direction 😄!