## Summary

- Document the new ChatGPT Subscription provider from #53166
- Add to the LLM Providers page alphabetically between Anthropic and
DeepSeek
- Cover sign-in flow, available models (GPT-5.5, GPT-5.3 Codex), and
sign-out

## Test plan

- [ ] Verify section renders correctly in docs preview
- [ ] Confirm links and anchors work

Release Notes:

- N/A