Document direct install commands

[efrazer-oai]

Summary

• document the new direct install commands in the root README and install guide
• point the documented commands at https://chatgpt.com/codex/install.sh and https://chatgpt.com/codex/install.ps1
• update the install requirements copy to treat native Windows as supported

Dependencies

• Depends on Add Windows direct install script #12741
• Depends on https://github.com/openai/openai/pull/717915

Testing

• docs-only change

[github-actions]

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