Add OpenAI Codex setup documentation to MCP reference

[danlister]

📋 Description

Added OpenAI Codex setup to Umbraco MCP reference material.

📎 Related Issues (if applicable)

N/A

✅ Contributor Checklist

I've followed the Umbraco Documentation Style Guide and can confirm that:

• Code blocks are correctly formatted.
• Sentences are short and clear (preferably under 25 words).
• Passive voice and first-person language (“we”, “I”) are avoided.
• Relevant pages are linked.
• All links work and point to the correct resources.
• Screenshots or diagrams are included if useful.
• Any code examples or instructions have been tested.
• Typos, broken links, and broken images are fixed.

Product & Version (if relevant)

Umbraco MCP

Deadline (if relevant)

N/A

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[eshanrnh]

Thanks for the PR, @danlister 🙌 Can you also add an entry to the new article in the Summary.md file so that it shows up in the Docs site?