Add demo for locale plugin configuration

[KeremAR]

This PR adds a missing demo for Locale plugin configuration.

• Add README with locale plugin examples
• Add locale.yaml with minimal configuration
• Demonstrates systemLocale and ignoreAcceptLanguage settings

The Locale plugin is commonly used to force Jenkins UI language (e.g., always use English regardless of browser settings), but there was no demo in the demos/ folder. This addition provides examples for the appearance:locale configuration.
issue 2748

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[timja]

Follow the contributing guide, https://github.com/jenkinsci/configuration-as-code-plugin/tree/master/demos#contributing

• all changes need a test
• The demos/locale/locale.yaml file isn't needed