Add API Center MCP registry URL

[luizbon]

Why: The API Center MCP registry URL is not documented anywhere, adding it to the documentation to help with the setup.

Closes: #40894

What's being changed (if available, include any code snippets, screenshots, or gifs):

Adding an example URL to the notes section after the API Center configuration steps.

Check off the following:

• [ ] A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• [ ] The changes in this PR meet the docs fundamentals that are required for all content.
• [ ] All CI checks are passing and the changes look good in the review environment.

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Table of review links

Note: Please update the URL for your staging server or codespace.

The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
copilot/how-tos/administer-copilot/configure-mcp-server-access.md	fpt ghec	fpt ghec

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

🤖 This comment is automatically generated.

[Sharra-writes]

@luizbon Thanks for opening a PR and pull request! This looks fairly simple, so I'll look into it and get back to you.

[trirawat131201-cpu]

https://github.com/github/docs/issues/40894#issuecomment-3576519920