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.

[welcome]

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[Copilot]

Pull Request Overview

This PR adds documentation for the Azure API Center MCP registry URL to help users with setup and configuration. The change provides a clear example of how to construct the correct URL format for accessing the MCP Registry through Azure API Center.

• Adds clarification on the Azure API Center URL format for MCP Registry access
• Includes a concrete example URL pattern with placeholders for user-specific values

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[github-actions]

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Table of review links

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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

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[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.