Add code owners syntax page to left nav

Add navigation item to global navigation

• Link to the merge request that introduced the new page: Move CODEOWNERS syntax reference to a child page (gitlab!117464 - merged)
  • https://docs.gitlab.com/ee/user/project/codeowners/reference.html will be the page as soon as the docs site updates
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page: https://aqualls-add-codeowners-syntax-page.docs.gitlab-review.app/ee/user/project/codeowners/reference.html

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)