Resolve "Document how to add a new model for model selection"

What does this MR do and why?

This change adds comprehensive documentation for GitLab's Model Switching Framework, which allows customers to choose between different AI models for various features. The documentation explains how developers can add new AI models to the system by defining them in configuration files and making them available for users to select. It covers the two-step process: first defining the model with its settings and parameters, then making it selectable for specific features. The guide includes examples, validation requirements, and testing instructions to ensure new models work properly once added to the system.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

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Related to #572823 (closed)