Updated the extension registry subpages order

[Luuk1983]

📋 Description

I recently updated the extension registry pages and I noticed that I forgot to change the order of the pages in the menu. The idea is to read them in this order:

Introduction > Register an extension > replace, exclude or Unregister. This is clear if you check the content of the overview page:

(https://docs.umbraco.com/umbraco-cms/customizing/extending-overview/extension-registry)

However, I did not update the menu:

This PR fixes this.

📎 Related Issues (if applicable)

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✅ 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)

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Deadline (if relevant)

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📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide

[eshanrnh]

Looks great, @Luuk1983 🙌 I added a small suggestion to capitalize the title.