docs: update Windows LLVM setup

[hydai]

Explanation

This PR updates the Windows source-build instructions to match the current WasmEdge Windows CI setup. The previous page still pointed users at the older LLVM 16.0.6 Windows package and documented a manual Visual Studio Enterprise-to-Community path edit for diaguids.lib in LLVMExports.cmake.

The current CI uses the WasmEdge llvm-windows 21.1.3 package. I also verified the downloaded 21.1.3 asset locally and confirmed its exported CMake metadata no longer contains a hard-coded Visual Studio Enterprise/Community DIA SDK path.

Related issue

Fixes WasmEdge/WasmEdge#2423.

What type of PR is this

/kind documentation

Proposed Changes

• Update the Windows build instructions to LLVM 21.1.3 and Windows SDK 26100.
• Remove the obsolete manual LLVMExports.cmake workaround.
• Use Invoke-WebRequest in the localized PowerShell download example.

This contribution was developed with assistance from OpenAI Codex.

Validation

• git diff --check
• npx prettier --check docs/contribute/source/os/windows.md i18n/zh/docusaurus-plugin-content-docs/current/contribute/source/os/windows.md
• npm run build