Rearrange Gitaly-related global nav items

Renames topic titles in menu to match: gitlab!57448 (merged), also moves configure topics underneath main Gitaly and Gitaly Cluster topic.

• Before: https://docs.gitlab.com/ee/administration/gitaly/
• After: http://eread-rearrange-gitaly-global-nav-items.178.62.207.141.nip.io/ee/administration/gitaly/