Add 'Migrations for multiple databases' to the nav bar
https://docs.gitlab.com/ee/development/database/migrations_for_multiple_databases.html doesn't have a spot in the navigation menu. This MR adds it in order to make the page more discoverable
https://docs.gitlab.com/ee/development/database/migrations_for_multiple_databases.html doesn't have a spot in the navigation menu. This MR adds it in order to make the page more discoverable