docs: add Router One model provider guide

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Description

Adds documentation for configuring Router One with Continue through Continue's existing OpenAI-compatible provider path.

Changes

• add a Router One provider page with YAML and deprecated JSON examples
• document pinned model configuration and optional model: auto routing
• add Router One to the model providers overview and More Providers sidebar
• include API key, model catalog, and troubleshooting links

No new provider implementation is required because Router One uses Continue's existing provider: openai client with apiBase: https://api.router.one/v1.

Validation

• npx --yes mintlify@4.2.454 validate
• parsed the YAML and JSON config examples
• verified openai/gpt-5.4-mini is present in the live Router One catalog with tool-calling support
• verified all added Router One links return HTTP 200
• git diff --check

Mintlify validation passed. The repository's broken-link check still reports six existing /CONTRIBUTING links in guides/continue-docs-mcp-cookbook.mdx; this PR adds no broken links.

Screen recording or screenshot

N/A — documentation-only change.

Checklist

• I've read the contributing guide
• The relevant docs have been updated
• This is a documentation-only change

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I have read the CLA Document and I hereby sign the CLA