Documentation: Add docs for dynamic model fetching feature

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Summary

PR #12046 added support for dynamically fetching provider models. This is a significant new feature that allows users to:

• Auto-load models for ollama and openrouter providers
• See a refresh button to fetch provider models after entering an API key
• Have fetched models merged and deduplicated with hardcoded models
• Persist contextLength, completionOptions.maxTokens, and capabilities to config.yaml when adding a model

Documentation Needed

• Document how the dynamic model fetching works
• Explain the refresh button functionality in the Add New Model UI
• Document which providers support dynamic fetching (ollama, openrouter, anthropic, gemini, and others via listModels())
• Explain how fetched model properties are persisted to config

Related PR

• feat: fetch provider models dynamically and fix isOpenSource pollution #12046: feat: fetch provider models dynamically and fix isOpenSource pollution

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Co-Authored-By: Continue noreply@continue.dev
Co-authored-by: bekahhw rhawrot@gmail.com