docs: Document OpenAI account authentication

[EtienneLescot]

Summary

Documents the new OpenAI Account authentication option for the OpenAI credential and the OpenAI Chat Model node.

This covers:

• the new OpenAI Account (ChatGPT) credential option
• the device code login steps
• the OpenAI Chat Model Authentication parameter
• the default model used with OpenAI Account authentication

Dependency

This PR depends on n8n-io/n8n#29184. It should merge after the code PR that adds OpenAI Account authentication support.

Closes #4526.

Validation

• git diff --check

I couldn't run the local MkDocs preview because mkdocs isn't installed in this checkout.

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Summary by cubic

Documented the new OpenAI Account (ChatGPT) authentication for the OpenAI credential and the OpenAI Chat Model node. Added device code login steps, clarified the Authentication parameter options, and noted that account auth loads models from your ChatGPT/OpenAI account with gpt-5.4-mini as the default.

^{Written for commit 2edc3bc. Summary will update on new commits. Review in cubic}

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