Rename navigation entry to use for new page

Companion to: gitlab!72918 (merged), repurposes an old global nav item for a new page.

Without an entry, the new page doesn't show up on the navigation:

• Before: https://docs.gitlab.com/ee/user/project/merge_requests/conflicts.html
• After: http://eread-rename-navigation-item.178.62.207.141.nip.io/ee/user/project/merge_requests/conflicts.html

CC @msedlakjakubowski