Add group-level protected branches API page to left nav

Add navigation item to global navigation

• Link to the merge request that introduced the new page: Add Group protected branches API (gitlab!110603 - merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:

Related links

• Related to Group-level protected branches - REST API (gitlab#389442 - closed) (introduces this page)
• Related to Group-level protected branches MVC (&8679) (parent epic)
• Other merge requests related to the feature:
  • Add database and model support for "Group-level... (gitlab!90040 - merged)
  • Group-level protected branches - All protected ... (gitlab!106532 - merged)