Document dynamic model fetching feature

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Summary

PR #12046 added support for dynamically fetching provider models. This allows users to see available models from their providers by clicking a refresh button after entering their API key.

What needs to be documented

1. Feature overview: After entering an API key in the Add Model form, users can click a refresh icon to "update models list using my API key"
2. Supported providers: Ollama auto-loads models, OpenRouter uses its public API, Anthropic calls /v1/models, Gemini uses v1beta/models, and other providers use their listModels() methods
3. Model metadata: The feature automatically detects and persists contextLength, maxTokens, and capabilities (tool_use, image_input) to config.yaml
4. Location: This should likely be added to docs/customize/models.mdx or a new section in the Add Model documentation

Related PR

#12046

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Co-Authored-By: Continue noreply@continue.dev