docs: add troubleshooting guide for OpenCV GUI backend errors

[ruchithakp6364]

This PR adds a troubleshooting guide for common OpenCV GUI backend errors
frequently encountered by Python users, including:

• ImportError: libGL.so.1
• HighGUI backend not available
• cv2.imshow() not implemented
• Assertion failed errors

These issues occur often in headless environments such as CI/CD systems,
Docker, Colab, servers, and WSL.

The guide includes:

• Explanations of why GUI errors occur
• Fixes for Linux, Windows, Docker, CI/CD, and WSL
• When to use opencv-python vs opencv-python-headless
• Example commands for installing missing system dependencies

This documentation aims to help users quickly diagnose and resolve GUI-related
OpenCV issues.

[ruchithakp6364]

Hello!
This PR adds a dedicated troubleshooting guide for common OpenCV GUI backend errors
(libGL.so.1 missing, imshow not working in CI, headless environments, Docker, WSL, etc.).

I tested the steps on multiple environments (Linux Desktop, WSL, CI, Docker),
and the guide should help beginners quickly identify the root cause.

Happy to modify or improve anything if needed. Thank you!