Add Aspire CLI pivot to upgrade guide with aspire update instructions

[Copilot]

Summary

This PR adds a new "Aspire CLI" pivot option to the upgrade guide (docs/get-started/upgrade-to-aspire-9.md), providing users with an alternative method for upgrading their Aspire projects using the aspire update command.

Changes

1. Zone Pivot Configuration

Added aspire-cli as a new pivot option to the dev-environment zone pivot group in docs/zones/zone-pivot-groups.yml. Users can now select "Aspire CLI" alongside the existing Visual Studio, VS Code, and .NET CLI options.

2. Upgrade Guide Documentation

Added two new Aspire CLI pivot sections to docs/get-started/upgrade-to-aspire-9.md:

Section A: "Use the Upgrade Assistant"

• Step-by-step instructions for installing the Aspire CLI using both Bash and PowerShell
• Instructions to navigate to the solution directory
• Detailed aspire update command usage with explanation of its capabilities:
  • Automatically locates the AppHost project
  • Prompts for channel selection (stable, preview, daily)
  • Detects outdated packages and computes safe upgrade targets
  • Updates package versions and NuGet.config if needed
  • Asks for confirmation before applying changes
• Includes the aspire-update.gif animation from the whats-new document
• Preview feature warning with recommendations to use version control

Section B: "Update the NuGet packages"

• Notes that aspire update handles package updates automatically
• Provides alternative manual update instructions for edge cases
• Links to CLI reference documentation

Benefits

• Provides a streamlined upgrade path for users who prefer command-line tools
• Reuses existing content and screenshots from the whats-new documentation
• Maintains consistency with existing pivot sections in structure and style
• Makes the new aspire update command more discoverable to users looking to upgrade

Related Documentation

• Installation instructions follow the same pattern as shown in docs/whats-new/dotnet-aspire-9.5.md
• Links to docs/cli/install.md for CLI installation details
• Links to docs/cli-reference/aspire-update.md for command reference

Fixes dotnet/docs-aspire issue regarding missing Aspire CLI upgrade instructions.

Original prompt

The upgrade-to-aspire-9 page is missing content which explains how to use aspire update to update an existing Aspire code base to the latest version of Aspire.

The page has a pivot at the time for Visual Studio Code, Visual Studio, and .NET CLI. We should add a new pivot for Aspire CLI which explains how to acquire the aspire CLI using the same instructions from the whats new in aspire 9.5 page and then instructs the user to go into their code base and run the aspire update command. It can reuse the screenshot from the "New aspiure update command (preview)" section in the whats new document.

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Internal previews

📄 File	🔗 Preview link
docs/get-started/upgrade-to-aspire-9.md	docs/get-started/upgrade-to-aspire-9
docs/zones/zone-pivot-groups.yml	docs/zones/zone-pivot-groups

[mitchdenny]

Slop :(

[IEvangelist]

Instead of adding to the existing pivot, we were going to create a new one, that removes .NET CLI and VS and orders it as Aspire CLI, Visual Studio Code. Then we could work on moving over content selectively. This change breaks existing articles that rely on this pivot as they now have an Aspire CLI option with no content for that pivot.