Docs: Document dynamic model fetching feature

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Summary

PR #12046 added support for dynamically fetching provider models, but this feature is not documented anywhere.

What needs to be documented

• How to use the refresh button to fetch available models from providers
• Supported providers: Ollama, OpenRouter, Anthropic, Gemini, and others with listModels()
• How fetched models are merged with hardcoded models
• What properties are auto-detected: contextLength, maxTokens, supportsTools
• How these settings are persisted to config.yaml

Suggested location

• docs/customize/model-roles/chat.mdx (already touched by the PR) or
• A new section in getting-started or customize docs

User benefit

Users can now dynamically discover and add models from their providers without manually looking up model IDs and capabilities.

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