Clarify that Expand-Archive requires PowerShell on Windows install guide

### Page URL

https://docs.flutter.dev/install/manual

### Page source

https://github.com/flutter/website/blob/main/sites/docs/src/content/install/manual.md

### Describe the problem

In the Windows installation guide under the extraction step, the documentation provides the following command:

PowerShell
Expand-Archive -Path -DestinationPath
However, the page does not specify that this is a PowerShell cmdlet. Users running standard Command Prompt (cmd.exe) receive an error stating that 'Expand-Archive' is not recognized as an internal or external command.

### Expected fix

To improve the onboarding experience for Windows users:

Explicitly label the terminal prompt as PowerShell (e.g., using a PS > prompt indicator or a note stating Run this in PowerShell).

(Optional) Add a Command Prompt alternative using tar, which is natively available on modern Windows versions:

DOS
tar -xf -C

### Additional context

_No response_

### I would like to fix this problem.

- [ ] I will try and fix this problem on docs.flutter.dev.

Open sites/docs/src/content/install/manual.md and start at the Windows extraction step. Clarify that Expand-Archive must be run in PowerShell, and review whether the optional tar alternative belongs there. Done when the rendered installation guide clearly identifies the required shell and no longer leads Command Prompt users to an unrecognized-command error.

Written by the indexing model from the issue text.