docs: add Atomic Chat local provider setup guide

Proposal

Document using Atomic Chat as a custom OpenAI-compatible provider in ~/.codex/config.toml.

Draft docs branch: https://github.com/yanalialiuk/codex/tree/docs/atomic-chat-integration

model_provider = "atomic-chat"
model = "<model-id-from-/v1/models>"

[model_providers.atomic-chat]
name = "Atomic Chat"
base_url = "http://127.0.0.1:1337/v1"
wire_api = "chat"
env_key = "ATOMIC_CHAT_API_KEY"

export ATOMIC_CHAT_API_KEY=atomic-chat-local

Compare / open PR: https://github.com/openai/codex/compare/main...yanalialiuk:codex:docs/atomic-chat-integration

Works with Codex CLI and the desktop app (same config file).