Add example about not repeating code

What does this MR do and why?

Adding this example helps reducing duplicated code in Anthropic responses, at least for claude-2, didn't help much for claude-instant-1 though.

See #425875 (closed)

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In rails console enable the experiment fully

   Feature.enable(:code_completion_anthropic)

2. then assuming you have Gitlab extension plugin enabled in your IDE (https://docs.gitlab.com/ee/user/project/repository/code_suggestions/#supported-editor-extensions) and code suggestions enabled, you can try it in situations when completion is triggered in a new empty methods

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.