Add section on updating Copilot CLI

[kaviraj07]

Why:

Closes: #44459

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

Added a new "Updating Copilot CLI" section to content/copilot/how-tos/copilot-cli/set-up-copilot-cli/install-copilot-cli.md.

The section briefly explains that users can update the CLI in two ways:

• /update — slash command from within an active Copilot CLI session
• copilot update — from the shell, without launching the CLI

It links to the canonical CLI command reference rather than duplicating the full command list.

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

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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/copilot-cli/set-up-copilot-cli/install-copilot-cli.md	fpt ghec	fpt ghec

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

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