Add authentication instructions for device auth flow in Codex setup guide

Discord Message

Author: perplamps Channel: #feedback Link: https://discord.com/channels/1405685085923049482/1405686161791516873/1456059011836285083

it'd be a good idea to provide some instructions on how to auth codex, as the default account-based flow does a localhost callback that doesn't work (unless I'm missing something)

seems to require toggling a security setting in openai settings for device auth flow and then using codex login --device-auth in the shell