Restructures code coverage docs

What does this MR do and why?

Reorganizes and consolidates code coverage documentation to make it clearer and more user-friendly. This MR:

• Combines content from multiple pages into a single page
• Clearly distinguishes between coverage keyword and coverage report features
• Improves organization and readability
• Updates file structure - adds new code_coverage folder and files and adds redirects:

  testing/
  | code_coverage/
  |---cobertura.md
  |---_index.md    (formerly testing/code_coverage.md)
  |---jacoco.md

Changes made

• Added clear sections for the two main methods for code coverage
• Grouped regex patterns by language using tabs
• Integrated test coverage visualization content, including information about child pipelines and data retention
• Removes outdated screenshots
• Updated crosslinks and related topics
• Improved formatting and readability

Review app

Code coverage page: https://main-ee-179914.docs.gitlab-review.app/ee/ci/testing/code_coverage/index.html

Files consolidated

Content from these pages has been combined into a page (in code_coverage.md):

• Code coverage
• Test coverage visualization

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• Docs: Code coverage documentation restructure (#516089 - closed)
• Improve documentation for Code Coverage (#354906 - closed)
• Moves tool-specific docs as child pages for cod... (gitlab-docs!5345 - merged)

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