Added several orphan topics to global nav

This MR adds several orphaned topics to the Global Navigation.

• https://docs.gitlab.com/ee/install/openshift_and_gitlab/index.html Removed from this MR

• https://docs.gitlab.com/ee/install/relative_url.html

• https://docs.gitlab.com/ee/downgrade_ee_to_ce/index.html

• https://docs.gitlab.com/ee/raketasks/sidekiq_job_migration.html

• https://docs.gitlab.com/ee/gitlab-basics/index.html

• https://docs.gitlab.com/ee/install/postgresql_extensions.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.