docs: add example config.toml (#5175)

I was missing an example config.toml, and following docs/config.md alone
was slower. I had GPT-5 scan the codebase for every accepted config key,
check the defaults, and generate a single example config.toml with
annotations. It lists all keys Codex reads from TOML, sets each to its
effective default where it exists, leaves optional ones commented, and
adds short comments on purpose and valid values. This should make
onboarding faster and reduce configuration errors. I can rename it to
config.example.toml or move it under docs/ if you prefer.

README.md

@@ -81,6 +81,7 @@ Codex CLI supports a rich set of configuration options, with preferences stored
   - [Custom prompts](./docs/prompts.md)
   - [Memory with AGENTS.md](./docs/getting-started.md#memory-with-agentsmd)
 - [**Configuration**](./docs/config.md)
+  - [Example config](./docs/example-config.md)
 - [**Sandbox & approvals**](./docs/sandbox.md)
 - [**Authentication**](./docs/authentication.md)
   - [Auth methods](./docs/authentication.md#forcing-a-specific-auth-method-advanced)