Improve organization of code coverage documentation

What does this MR do?

The code coverage docs are spread in areas that are not obvious to find. The MR focuses on organizing code coverage features in a single spot along with cleaning up pointers.

• Pipelines Settings Page: Removed lonely code coverage section that didn't seem to fit and placed in new "Code Coverage" page under testing directory
• Approvals Page: The code coverage rule was under "Settings" that seemed out of place. Moved to "Rules" page along with other rules. Also moved Troubleshooting section to bottom.
• Code Coverage Page (New): Curate all the related code coverage features:
  • View in MR
  • View project history
  • View group history
  • Badge
  • Code Coverage Approval Rule
• CI YAML page: Reference parse examples
• Navigation update: gitlab-docs!3745 (merged)

Related issues

#385103

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
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  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.