docs: update Tailwind CSS setup guide for v4 (2025) with legacy v3 support.

[Roshan-Chourasia]

This PR updates the Tailwind CSS setup guide to reflect the latest v4 installation process (2025) while preserving the original v3 steps used in the Sigma Web Development Course.

🔧 Changes made:

• Added Tailwind CSS v4 (2025) setup instructions with clear steps
• Retained the v3 instructions under a legacy section for compatibility
• Included a note to help learners avoid confusion due to version differences in the video

This update helps new learners get started with the latest Tailwind version, while still supporting those following the original (older) course videos.

[Roshan-Chourasia]

Hi! This update is to help future learners facing Tailwind v3/v4 confusion. Would love your feedback!