Make coverage approval rule frontend-configurable

What does this MR do?

For #331001 (closed)

This MR adds a configurable MR approval rule (Coverage-Check) to project settings.

The backend for this rule was already implemented in !63079 (merged), which works with any rule named Coverage-Check. This MR just makes it easier to discover and configure a rule with the required name.

This MR also adds documentation about what the rule does and how to configure it.

Handy test project: https://gitlab.com/iamricecake/test-merge-approvals

Screenshots

screenshot	explanation
	unconfigured placeholder Coverage-Check rule in project settings (Settings -> General -> MR Approvals)
	configuring a Coverage-Check rule
	configured Coverage-Check rule
	once the rule is defined, an MR that causes coverage to go down will not be mergeable until it is approved by the user/group
	and an MR that doesn't affect coverage will not require the same approval

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)