[Docs] Add Windows installation instructions

[louiswang524]

Adds platform-specific virtual environment activation instructions for Windows users.

Changes:

• Added Windows PowerShell activation command (.venv\Scripts\activate)
• Added Windows Command Prompt activation command (.venv\Scripts\activate.bat)
• Separated installation steps for Linux/macOS vs Windows
• Clarified platform differences in Environment Setup section

Issue: The existing documentation only showed Unix/Linux commands (source .venv/bin/activate), which doesn't work on Windows. This caused confusion for Windows users trying to activate the virtual environment.

Benefits:

• Windows users can now follow clear, platform-specific instructions
• Reduces installation friction and support questions
• Provides both PowerShell and Command Prompt options for Windows
• Maintains existing Unix/Linux instructions