Add missing administration topics to global nav

This MR adds three missing topics to the global navigation

• https://docs.gitlab.com/ee/administration/raketasks/smtp.html

• https://docs.gitlab.com/ee/administration/operations/extra_sidekiq_routing.html

• https://docs.gitlab.com/ee/administration/libravatar.html

• Link to the merge request of the new page:

• Give the MR a descriptive title.

• Link to the merge request that introduced the new page.