Add developer reference examples for extending AI Experiments

[jeffpaul]

Provide concrete developer examples showing how to extend the AI Experiments plugin. This should include at least one example of creating a custom experiment using the existing APIs and patterns.

These examples should position the plugin as a reference implementation for AI in WordPress.

Acceptance criteria:

• At least one documented example of a custom experiment
• Example follows recommended patterns and APIs
• Documentation is easy to follow for experienced WordPress developers
• Example is clearly marked as reference/demo code