docs: clarify codex provider and platform key setup

[kgskr]

Summary

• clarify Codex CLI provider setup so users know model_provider goes at the top of ~/.codex/config.toml
• show that the [model_providers.codex-lb-cinamon] block should be pasted at the bottom of the file
• add a Platform API key registration guide, including the three Eligible routes checkboxes required for Codex app / Codex CLI usage

Details

• remove the misleading implication that model and model_reasoning_effort must be rewritten just to use codex-lb-cinamon
• keep the API-key-auth variant example, but frame it as an extension of the same provider block
• document the dashboard-side Platform key prerequisites and route-family selection in plain language

Validation

• not run (README-only change)