Add to Derive Reference an example of what a derive exapnds to

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Clap Version

3.1.0

Describe your use case

In the derive reference, we try to explain the principles for how the derive works so people can use any part of the builder documentation as part of the derive but we still frequently get questions on how to use a feature.

Describe the solution you'd like

A more concrete example of the principles will hopefully help.

We shouldn't just run cargo expand but show it as idiomatic.

We should include

• Command methods
• Arg methods
• Subcommands
• PossibleValue methods

Alternatives, if applicable

No response

Additional Context

No response