Create new Git Advanced operations page

What does this MR do and why?

This MR creates a new Advanced operations parent page. It will be nested in the Use Git section. This is part of a larger structural change.

It will include the following child pages:

- Advanced operations
   - Rebase and resolve conflicts
   - Undo changes
   - Tutorial: Update Git remote URLs

Review app

• Advanced Git operations

Related issues

• Issue: #499459 (closed)
• Child item: #499463 (closed)
• MR to update navigation: Add advanced operations nav item (gitlab-docs!5144 - merged)