fix(docs): replace curl command with PowerShell-compatible Invoke-WebRequest

[ShigrafS]

Explanation

The current Windows build instructions for WasmEdge recommend using the curl -sLO command to download LLVM. However, this command fails in PowerShell because Windows aliases curl to PowerShell's Invoke-WebRequest, which does not support the -sLO flags. This PR updates the instructions with a PowerShell-compatible command to ensure smooth downloads on Windows, improving the developer experience.

Related issue

Closes: WasmEdge/www#31

What type of PR is this

/kind documentation

Proposed Changes

• Replaced the curl -sLO command in Windows build instructions with a PowerShell-compatible Invoke-WebRequest command.

[juntao]

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[ShigrafS]

@hydai
Proceeding with Invoke-WebRequest since this is the most predictable, native, and robust option for Windows users, especially those who haven't installed curl.exe or don't understand PowerShell aliasing or are not familiar with Unix-style tools.