Move protected branches file

What does this MR do and why?

• Rename the protected-branches.md file to protected.md and move it.

  • Current location: doc/user/project/protected_branches.md
  • Move to: doc/user/project/repository/branches/protected.md

• Update all links from the old URL (relative path) to the new one.

Reason for the change

Inside the project directory, there is already repository/branches that includes Branches and Default branch .md files. It makes sense, from a consistency standpoint, to include the Protected branches file. This is also how it's reflected in the left navigation:

Related

• Issue: Review Source Code documentation pages (#480482 - closed)
• MR comment for this update: !164517 (comment 2088352874)
• MR to update navigation item: Draft: Update protected branch navigation URL (gitlab-development-kit!4008 - closed)