Use code coverage graphql API

What does this MR do?

Related to #267624 (closed)

Switch from using a client resolver to using the actual GraphQL API that was implemented in !44472 (merged).

Added some docs explaining the feature and organizing the Repositories Analytics page.

This feature is behind a feature flag, so no changelog is required.

Screenshots (strongly suggested)

The UX is mostly the same as before, except for the following:

• When a project is selected that doesn't have any code coverage, we default to 0.
• Added a section header

Before	After

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done