Docs: Document dynamic model fetching feature

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Summary

PR #12046 added support for dynamically fetching provider models, which is a significant user-facing feature with no documentation.

Feature Details

• Users can now dynamically fetch models from providers using their API key
• A refresh icon appears after entering an API key to "update models list using my API key"
• Supported for:
  • Ollama (using tags API - auto-loads)
  • OpenRouter (public API - auto-loads)
  • Anthropic (/v1/models)
  • Gemini (v1beta/models)
  • Other providers via listModels() methods
• Results include context length, max tokens, and tool use capabilities
• Fetched models are deduplicated with hardcoded models

Documentation Needed

1. Add a section to the Add Model documentation explaining the dynamic model fetching
2. Document which providers support this feature
3. Explain how the refresh button works and when it appears
4. Document the auto-loading behavior for Ollama and OpenRouter

Related PR

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

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