Moves tool-specific docs as child pages for code coverage

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• Link to the merge request that introduced the new page: gitlab!179914 (merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page: https://lyspin-reorg-code-coverage-docs.docs.gitlab-review.app/ee/ci/testing/code_coverage/_index.html

Current nav (before)

Nav changes (after)

Testing/
└── Code coverage/                              # Main coverage documentation
    ├── Cobertura coverage report/                     # Cobertura-specific setup and features
    └── JaCoCo coverage report /                        # JaCoCo-specific setup and features

# This structure:
 # - Makes code coverage the main entry point
 # - Moves tool-specific docs as child pages
 # - Removes the Test coverage visualization page 
 # - Follows a logical hierarchy from general to specific