docs: add json-based example for the extending expressions article

[petar-i-todorov]

No description provided.

[Copilot]

Pull Request Overview

This PR adds a JSON-based configuration example to the extending expressions documentation article. The changes demonstrate how to configure assembly references using JSON format in addition to the existing XML configuration approach.

• Adds JSON configuration example alongside existing XML example
• Updates link reference to use proper slug format
• Corrects XML element capitalization for consistency

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