docs: Add documentation for dynamic model fetching feature

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Summary

PR #12046 merged a feature that allows users to dynamically fetch provider models using the models/fetch protocol. This feature:

• Auto-loads models for ollama and openrouter providers
• Adds a refresh button that appears after entering an API key to fetch available models from providers
• Supports fetching models from anthropic, gemini, and other providers using their listModels methods
• Persists contextLength, maxTokens, and capabilities to config.yaml when adding a model

Documentation Needed

The documentation should cover:

1. How to use the refresh/fetch models feature in the Add Model UI
2. Which providers support dynamic model fetching
3. What model properties are automatically detected and persisted
4. Any limitations or considerations when using this feature

Suggested Location

• docs/customize/models.mdx - Add a section about discovering available models
• Or create a new guide under docs/guides/

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