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.

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Source	Review	Production	What Changed
copilot/how-tos/copilot-cli/set-up-copilot-cli/install-copilot-cli.md	fpt ghec	fpt ghec

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[Sharra-writes]

@kaviraj07 Normally I would say that this should be added to "next steps" at the bottom, since we're trying not to duplicate information. The article you're linking to is so without context, though, that this addition might be reasonable. I'm going to check with the CLI team to see what they think.

[Sharra-writes]

@kaviraj07 Okay, the CLI team does want it noted where the update command can be found, but this article is explicitly about installing for the first time, so the information on updates does belong at the bottom under "next steps." Something like, "For a list of commands for {% data variables.copilot.copilot_cli_short %}, including the command to update, see [AUTOTITLE](/copilot/reference/copilot-cli-reference/cli-command-reference)."