docs: add Codex setup instructions

[leochiu-a]

Summary

This PR adds setup instructions for Codex users to the README documentation. Codex is a new AI coding assistant that supports MCP (Model Context Protocol) servers, and this change makes it easier for Codex users to integrate the Notion MCP server into their workflow.

Key Changes

• Added a new "Codex" section to the README with configuration instructions
• Provided TOML configuration example for ~/.codex/config.toml
• Documented how to use npx to run the Notion MCP server with Codex
• Included environment variable setup example for NOTION_TOKEN

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• ♻️ Refactoring (no functional changes)
• 🎨 Style/formatting changes
• 🧪 Test improvements
• 🔧 Configuration changes

Breaking Changes

None

Checklist

• 📝 Code follows the style guidelines
• 👀 Self-review has been performed
• 🧪 Tests have been added/updated
• 📖 Documentation has been updated