Update API Center URL instructions for allowlist policy

[OzanSimsekler]

Added information about appending '/workspaces/default' to API Center URL for allowlist policy.

Why:

Closes: N/A

What's being changed (if available, include any code snippets, screenshots, or gifs): Information added regarding how the URL should be changed for API Center to work with Github Copilot.

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.

[github-actions]

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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/manage-mcp-usage/configure-mcp-registry.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]

@OzanSimsekler Thanks for opening a PR! The placement of this information is probably not where we want it, but I can't find the information elsewhere, so I'm checking with the Copilot team to see if they think we need to document it. Provided they do, I'll take it back to docs and figure out where we would like to put it.