Add faces detection example

[SimakovSergey]

Description

Adding the new example for computer vision - faces detection. It is focused on a specific task of detecting faces with the bounding boxes together with indicating how to use it with polygons.

Connected issues (if any)

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Documentation and examples improvement (changes affected documentation and/or examples)

Checklist:

• [x] I have read the CONTRIBUTING document.
• [x] I hereby agree to the terms of the CLA available at: https://yandex.ru/legal/cla/?lang=en
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

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