Update URL to Code Owners page

Add navigation item to global navigation

• Link to the merge request that introduced the new page: gitlab!117061 (merged) moves an existing page
  • Old: doc/user/project/code_owners.md
  • New: doc/user/project/codeowners/index.md
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:

Related issues

• Related to Technical design doc for Code Owners (gitlab#219916 - closed)
• Related to Add code owner documentation details to explain... (gitlab#396315 - closed)