Removed multiple dead links

[alabhyajindal]

Description

Removed multiple dead links under sections such as React, Python, and openCV.

Motivation and Context

Clean up dead or outdated tutorials

Types of changes

• Content Update (change which fixes an issue or updates an already existing submission)
• New Article (change which adds functionality)
• Documentation change

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have made checks to ensure URLs and other resources are valid

[alabhyajindal]

@enkeyz Can you please have a look at this?

[enkeyz]

Under "OpenCV" category, you removed "Object Detection using Mask-R-CNN", which still works. Correct that mistake, and I'll accept your PR.

[enkeyz]

Thanks!