Refactor netbirdAPI key configuration

[mlsmaycon]

Updated netbirdAPI configuration to specify key name and key.

[Copilot]

Pull Request Overview

This PR refactors the netbirdAPI configuration in the Kubernetes operator documentation to use a more structured format for specifying secrets. Instead of providing just the secret name as a string, the configuration now requires both the secret name and the specific key within that secret.

• Updated netbirdAPI.keyFromSecret from a string value to an object structure
• Added explicit key specification ("NB_API_KEY") alongside the secret name

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