[IMPROVEMENT] Add WSL2 setup instructions for Windows users

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The current Windows installation section directs users to Cygwin, which
is the legacy method. The 2026 GSoC project goals for Sample Platform NG
specifically mention moving toward WSL2/Docker for Windows users.

This PR adds a WSL2 section above the existing Cygwin instructions,
giving Windows users a modern and recommended setup path.

Changes made:

• Added Windows (Recommended: WSL2) section to install/installation.md
• Includes step-by-step WSL2 setup instructions
• Notes that Cygwin is the legacy method