Add missing pages from Administration directory
Add navigation item to global navigation
The script for finding pages not in the nav identified these, so let's fix them:
- https://docs.gitlab.com/ee/administration/troubleshooting/test_environments.html
- https://docs.gitlab.com/ee/administration/troubleshooting/postgresql.html
- https://docs.gitlab.com/ee/administration/troubleshooting/linux_cheat_sheet.html
- https://docs.gitlab.com/ee/administration/troubleshooting/gitlab_rails_cheat_sheet.html
- https://docs.gitlab.com/ee/administration/troubleshooting/diagnostics_tools.html
and this standalone, which had an obvious home: