Add minimum coverage threshold (6/6): Documentation

What does this MR do and why?

This is MR 6 of 6 in a stacked series that adds a minimum coverage threshold to the Coverage-Check approval rule.

MR	Branch	Base	Content
1	6284-coverage-threshold-1-db-model-ff	master	DB migration & model validation (!226363 (merged))
2	6284-coverage-threshold-2-service-rest	6284-coverage-threshold-1-db-model-ff	Service logic, REST API & feature flag (!226370 (merged))
3	6284-coverage-threshold-3-graphql	6284-coverage-threshold-2-service-rest	GraphQL mutations & fields (!226372 (merged))
4	6284-coverage-threshold-4-blockage-reason	6284-coverage-threshold-3-graphql	blockage_reason feature & GraphQL exposure (!226394)
5	6284-coverage-threshold-5-frontend	6284-coverage-threshold-4-blockage-reason	Frontend: Settings UI + MR widget feedback (!226395)
6 (this MR)	6284-coverage-threshold-6-docs	master	Documentation (standalone, no dependency)

This MR replaces !224375 (closed). Splitting was discussed and agreed upon with @kherman1 and @hfyngvason in !224343 (closed).

This MR includes:

• Code coverage docs (doc/ci/testing/code_coverage/_index.md): Expanded "Add a coverage check approval rule" section with threshold configuration step, blocking conditions list, and MR widget feedback description
• API docs (doc/api/merge_request_approvals.md): Added coverage_minimum_threshold parameter to Create and Update project approval rule endpoint tables
• Approvals overview (doc/user/project/merge_requests/approvals/_index.md): Updated required approvals reference to mention minimum threshold

Files to review in this MR

• doc/ci/testing/code_coverage/_index.md - Code coverage documentation
• doc/api/merge_request_approvals.md - API reference
• doc/user/project/merge_requests/approvals/_index.md - Approvals overview

Related to #6284 Extracted from !224343 (closed)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

• The change is documentation only, no code changes.
• All three documentation files consistently describe the same feature behavior.
• API parameter documentation matches the implementation (type: float, optional, 0-100, code_coverage rules only).
• No auto-generated files modified (GraphQL reference and OpenAPI specs are updated in MR 2 and MR 3).

Related to #6284