Add Codex API key support and Gemini 2.5 Flash-Lite model documentation updates

- Documented Gemini 2.5 Flash-Lite model in English and Chinese README files.
- Updated README and example configuration to include Codex API key settings.
- Added examples for custom Codex API endpoint configuration.

config.example.yaml

@@ -41,6 +41,11 @@ generative-language-api-key:
   - "AIzaSy...03"
   - "AIzaSy...04"
 
+# Codex API keys
+codex-api-key:
+  - api-key: "sk-atSM..."
+    base-url: "https://www.example.com" # use the custom codex API endpoint
+
 # Claude API keys
 claude-api-key:
   - api-key: "sk-atSM..." # use the official claude API key, no need to set the base url