Add user levels configuration for custom agents

[ashis-baral]

Why:

Fix: #46157

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

Adds the missing user-level custom agent configuration location to the Where you can configure custom agents section of the GitHub Copilot CLI custom agents documentation.

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/concepts/agents/cloud-agent/about-custom-agents.md	fpt ghec	fpt ghec	from reusable
copilot/concepts/agents/copilot-cli/about-custom-agents.md	fpt ghec	fpt ghec	from reusable

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

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[subatoi]

Thanks for raising a new PR, @ashis-baral; I'll get this triaged for review by the appropriate team as soon as possible.

[ashis-baral]

@subatoi Thanks for the update!