Create E2E tests for minSubGroup and docs

[davidLif]

Create comprehensive end-to-end and unit test coverage for the minSubGroup feature.

Test single-level and multi-level hierarchies, validation rules, reclamation/preemption
scenarios, and backward compatibility.

Create user-facing docs with examples based on the design to explain how to use the new minSubGroup API.

New doc with examples in relevant doc folder

New E2E tests for testing the new API & backwards compatibility

[davidLif]

E2e tests implemented by #1428