docs: configure custom providers

[dianed-square]

Summary

This PR adds the "Configure Custom Provider" section to the "Support LLM Providers" topic and restructures two existing sections (h2 -> h3) to help clarify the relationship

• Move "CLI Providers" to H3 under "Available Providers" (they're a special subset of available providers)
• Move "Using Custom OpenAI Endpoints" to H3 under "Configure Provider and Model" (extends the built-in OpenAI provider)
• Add "Configure Custom Provider" as H2 (creates entirely new provider entries)

Type of Change

• [ ] Feature
• [ ] Bug fix
• [ ] Refactor / Code quality
• [ ] Performance improvement
• [x] Documentation
• [ ] Tests
• [ ] Security fix
• [ ] Build / Release
• [ ] Other (specify below)

AI Assistance

• [x] This PR was created or reviewed with AI assistance

Testing

Manual testing

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PR Preview Action v1.6.0 :---: Preview removed because the pull request was closed. 2025-12-11 22:45 UTC