Move `Group -> Members` to top-level, fix missing sub-nav for Subgroups

What does this MR do?

This MR is part of changes for #26348 (closed)

It also fixes #28678 (closed) which was caused by changes made in !9309 (merged)

Why was this MR needed?

Please see #26348 (closed)

Screenshots

TBA

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #28678 (closed)